Generally, bleeding isn't a good thing; what with the whole "bleeding to death" bit and there usually being an angry person nearby wanting to make you bleed some more. However, some have the ability to turn their lifeblood into the deathblood of their adversaries.
Whether it's a vampire, "blood mage," alien, mutant, demon or more exotic fare, this character can kill with Bloody Murder, the ability to use blood as an offensive weapon. Possible weapon mechanisms include:
Poison Blood: The blood itself is poisonous, and either direct contact or indirect exposure to it is enough to kill.
Josuke from Part 4 can use his Stand to snipe people with his blood.
Part 5's Risotto Nero, who could manipulate iron, including the iron in blood. This ended in such lovely images as Mr. Nero forming a pair of scissors in his enemy's throat, and forcing him to vomit up razor blades.
In Blood+, Chevalier can be killed only if exposed to the blood of the other Chiropteran Queen. It stops working after they become pregnant.
Saya also enters a state of increased strength, speed, and senses if she draw her own blood.
Alucard of Hellsing can make his blood, and indeed the spilled blood of others, do anything he wants. Anything. This includes absorbing the blood of his victims to claim their soul, all the way up to releasing literal tides of blood to spawn thousands-strong armies of said absorbed souls.
It also happens to be his greatest weakness: the character Schrodinger's ability is based on the paradoxical thought experiment he's named after; to be everywhere, and yet nowhere, reliant on his perception of self. Whilst he's normally completely invulnerable due to this, he commits suicide in volume 10 of the manga, and allows his body to be absorbed by Alucard, which results in him being unable to perceive himself amongst the literally millions of souls Alucard has claimed. By extension, Alucard becomes neither alive nor dead and ceases to exist.
Deadman Wonderland features a group of protagonists called Deadmen who possess the unique ability to kinetically control their own blood, which they primarily use to shape it into weapons—whips, bullets, you name it. The lead is the one with the bullets, which is annoying since the thing about bullets is they don't 're-use' very well, and Crow with his scythe can just keep slicing while Ganta has less and less blood in his body to work with.
Used in One Piece during the battle against Crocodile, who can turn into sand. His power doesn't work when he gets wet, so Luffy used the only thing he had handy; blood from the wounds he was receiving.
One guy in Trinity Blood has blood that instantly combusts when it comes it contact with air. The main character can also make a scythe out of his while in his Super Mode.
Vampires in Nightwalker can bite themselves and control their blood to form bladed weapons.
manga/Get Backers: Akabane once used the diamond dust he'd inhaled to create another one of his infamous scalpels. Anything that mixes with his bloodstream can be manipulated thusly. His Bloody Sword seems to actually be created from his blood.
Emishi also has flammable blood, and once used it to try to burn Shido to death. It's a desperate, last-chance attack, though; he usually sticks to his whip.
In Darker Than Black, the contractor Wei Zhi Jun is shown to have teleporting blood, which he uses offensively by cutting himself and splattering it on his opponents before he it activates it by snapping his fingers to take chunks of people and objects with it.
In Speed Grapher, Anti-Villain Suitengu Chouji's superpower allows him to control his blood in a variety of ways. He can create wings of blood that somehow support his weight, levitating drops of blood to throw at people, full-sized human puppets made of blood and make his blood sharp enough to cut people. On the other hand, he tries to not use it unless he can't win otherwise, since his body's need to rapidly heal itself over where the blood comes out brings his cells closer to cell death, thereby shortening his lifespan.
Bleach: Mayuri tends to fill Nemu's body full of poisons and various experimental drugs. In at least once case, when an enemy tried to use her body to regenerate himself, the poison was transferred to him with dire results.
Kiyoshi Mitarai, also known as Sea Man, had the ability to create water horrors by mixing some of his blood in it.
In addition, ninja demon Gama is able to make enchanted ink from his blood, which can paralyze enemies and lock their energy inside of their body, as well as, in outside material, make himself invisible. Kurama, after fighting him despite being partially paralyzed by his ink, makes the mistake of grievously injuring him, which, while causing him to die, weakens Kurama to an extreme degree due to the massive amount of blood that Gama spilled.
At one point in Flame of Recca, Tokiya Mikagami begins to run out of juice for Ensui, his water-based weapon, and uses his own blood to power it.
Towards the end of the manga, Hiruko of the Shishiten demonstrates the ability to control blood directly, absorbing it from his enemies and using it to heal himself and create crystalline weapons.
Soul Eater: The Black Blood can solidify in the body beneath attacks, protecting the body against blows, as seen used by both Crona and Maka when she gets under it's influence. Crona's black blood goes further, being the shapeshifting weapon partner, Ragnarok, who can still cut you up even after leaving the body in all directions. Black Clown is made of black blood as well and can merge with Crona for an additional power boost.
The Duel Monster Destiny Hero Bloo-D (how appropriate) in Yu-Gi-Oh! GX can rain down a swath of spears made out of blood spawned from his wings.
In Basilisk, Akeginu of the Iga Tsubagakure group not only is extremely seductive as well as deadly with her trustytantodagger, but she can also control her own blood, secreting it out of her pores to blind or tag her opponents or create a sort-of mist as she mixes it with the air in her surroundings.
Though it's not actually used as a weapon, Hyatt in Excel♥Saga has poisonous blood. Ingesting it will cause cramps and nausea, and it releases deadly gasses when evaporating. She also has a nasty habit of vomiting it all over the place.
In the manga/Tenjho Tenge manga, among the abilities Souichiro steals thanks to his power is a sort of 'water bullet', however Souichiro uses his own blood as a medium, and usually only after being wounded as opposed to going out of his way to bleed
Low-level Akuma in D.Gray-Man fire bullets made of their blood. It causes Body Horror and very quick death. Arystar Krory has blood that's immune to it. He can also infect Akuma with his own blood, which causes much the same effect as their blood has on humans. He can also make a marionette out of his own blood.
Boris Tepes Dracula from Shaman King uses blood as his spirit medium.
Nagi from the Mai Hi ME manga uses his own blood to summon Orphans.
Senri Shiki of Vampire Knight can form a long, telekinetically-controlled whip of blood able to slice through most living tissue.
Outenkun of Houshin Engi and his blood that melts metal and can turns into noxious, acidic gas.
Ginshu, the Miko from Amatsuki, has blood that is poisonous to demons. This prevents them from eating her/him (though it doesn't stop them trying).
Additionally, Orochimaru (or at least his true form) is eventually shown to have highly poison blood that evaporates into poisonous gas when spilled.
In an anime only scene, 6-tailed Kyuubi!Naruto seems to bleed fire.
Also of note is that at 4 tails, Kyuubi!Naruto's Battle Aura becomes such a strain on his body that it disintigrates his SKIN, at which point his blood seeps out and mixes with said battle aura, turning it opaque and blood-red.
Yuugami from Kekkaishi can turn his blood into weapons, as well as large, flying familiar-type creatures. Subverted in that he doesn't actually have any more blood than the average person and has to save it up for months before he has enough to do anything elaborate with it. He's also been shown reabsorbing blood stored for emergencies, so apparently he also has special red blood cells that can live for longer than the standard 3 or 4 weeks outside of the body.
Code:Breaker: The five Saint Fighters Aoba/Code:Revenger's goon squad all have this ability. This might be a sign that they're sisters, since the only other pair of people with the same power is a sister who shared it with her brother.
Fairy Tail: when Gray fights Ultear, she uses her time magic to melt all of his ice. Since Ultear's power only works on inanimate objects, he attacks her with some of his frozen blood.
Vamdemon/Myotismon from Digimon Adventure possibly has this, in the form of his Bloody Stream/Crimson Lightning attack, where he conjures a scarlet-red energy whip to attack.
His evolved form, BelialVamdemon, also has his Melting Blood attack, which dissolves enemies with blood from his shoulder cannons.
Tsukimiya from Bloody Cross can weaponize her blood, for example her "Blood Tepes/Blood Skewer" attack can make her spilled blood turn into spears that shoot up and impale her opponent.
Mirai from Kyoukai no Kanata uses her blood as a weapon, which she turns it into a sword and it is a unique power in the spirit world. However, due to her powers, she is being isolated from others, as well as the fact that expelling a sizable pint of blood can pass her out.
DC'sLobo had the ability to spawn a full-sized clone from a single drop of blood, though it was genetically removed by Vril Dox, his former employer in L.E.G.I.O.N. Different writers have seen fit to bring it back at their discretion.
Crimson Plague can use her blood as a very deadly weapon indeed; it will eat its way through just about anything and just a few drops are enough to turn her opponents into piles of fleshy goo. When she gets her periodher blood even spawns a virus that can kill a whole planet within a day. Add this to the fact that she's batshit insane and you have yourself one of the nastiest comic book characters around.
Captain Atom, when he's in Captain Atom form, bleeds energy when he's cut, as do Major Force and the Silver Shield (who share his origin). When the Silver Shield was first cut, the radiation release killed Damon Long and poisoned Heinrich Megala.
Nico Minoru of Runaways has a staff of power that only emerges if she is actively bleeding. Normally, this involves cutting herself (which is extra hilarious due to how goth she dresses), but she can call it forth at any time when she's on her period.
John Constantine of Hellblazer has demon blood as a result of a demon healing him. He specifically warns a child not to bite him at one point, because "you really don't want my blood in your mouth".
Then there's the time the King of the Vampires tries to drain Constantine. His jaw dissolves.
Spider-Man villain Carnage was created when an alien symbiote merged with the bloodstream of serial killer Cletus Cassidy. Mass-murder ensues.
Spider-Man's blood is radioactive, a fact that has caused the defeat of more than one vampire that has attacked him.
To go a little more in depth, should you feel great rage and recieve a Red Ring, you vomit out all your blood, which is then replaced by red light constructs of your bloodstream. Which means that any Red Lantern who overcomes the Unstoppable Rage the ring induces (with one exception) dies immediately.
Superman was once bitten by Dracula, but his solar-powered blood made the vampire lord explode.
The same thing occurs in Runaways, where Karolina, an alien from a race that absorbs energy from stars, accidentally kills a vampire by offering herself up as a Heroic Sacrifice.
The titular villain of Sword of Dracula wields telekinetic control over all the blood he has ever drained. His mastery is such that he assembles weapons, horses, minions and full-size castles with the stuff.
In With Strings Attached, the Brothers of Doom have a death touch, but they also have blood that will instantly kill anyone it touches, so when they expect invaders, they bleed into cups so they can throw it at people.
In Winter War, once the Barragan Fragment assumes a humanoid form again, its "age things to dust" power is reflected in its blood, which becomes the "corrosive blood" variant of this trope. The characters find this out the hard way, with one of them loosing a hand due to blood splatter when she managed to cut it.
In Fallout: Equestria an alicorn crystallizes her own blood into daggers to fight Littlepip. Pip later repurposes the technique (or at least its principles) to create blood-bandages for Xenith.
In Pages Of Harmony, the "Boiling Blood" variant occurs during a procedure that simulates conditions in space used by Twilight Sparkle to kill Fluttershy to extract Kindness.
In We Are All Pokémon Trainers, Constantin, a villainous psychic, can use telekinesis to form his exposed blood into a whip as a desperation weapon.
In Make a Wish, Harry's incredibly poisonous blood (it contains Basilisk venom) protects him several times. The first, when he is attacked by werewolf Fenrir Greyback, poisoning Fenrir while Harry completes the work with a silver knife. The second, when Lord Dracula gives him blood to turn him into a vampire (the only way he won't become a werewolf) but instead stabilizes things and turns Harry into some sort of hybrid. The third, when he gets bitten by several Australian poisonous spiders, who bite it when Harry's blood enters their mouthes.
As with John W. Campbell's original story Who Goes There?, on which it was based, this also turns out to be the monsters' fatal weakness, as it provides a foolproof method for telling them apart from the humans they're otherwise mimicking perfectly. Systematic testing of the entire base personnel and electrocution of the failures with an overclocked cattle-prod follows, forcing the remaining monsters to declare themselves on the spot. In the novella, unlike the film, the humans score an outright win and there is no downer ending.
In the movie Daybreakers, this becomes a major plot point near the end. In particular the cure to vampirism is spread by drinking blood from a cured vampire
In the Alien film series, the aliens have highly corrosive blood, making it very dangerous to wound them, especially while in space, because it is very likely to eat straight through the hull. The acid blood was created for the first film as an answer to the problem, "Why don't they just shoot it?" In later films, the aliens proved to find active uses for their acid blood. In Alien: Resurrection, two aliens kill a third to escape their holding cell. In Alien vs. Predator, an alien that gets its tail stinger chopped off proceeds to whip its wounded tail around and spray acid blood everywhere.
In the spinoff movie, Prometheus, the snake-like creatures bleed the same kind of acid-blood.
The Tales from the Crypt movie Demon Knight actually features two types of blood-based attacks. In one scene, the Collector, a powerful demon, cuts his hand and spills his green blood on the ground to form a small army of lesser demons, which rise from the blood itself. But blood was also his only weakness ? specifically, the blood contained in the Key (the film's MacGuffin). That's because the Key contained traces of the blood of Jesus Christ, mixed in with that of countless Guardians of the Key. The blood was not only deadly on contact to the Collector, but the protagonists could spill the blood in doors and windows to make it impossible for demons to enter; they would explode if they tried.
In X2: X-Men United, Mystique smuggles metal into the government's plastic prison (where Magneto is being held) by injecting a security guard with some sort of iron-rich solution.
In Subspecies, the vampire Radu's blood can spawn the titular creatures when spilled.
Played with in Saw: the two unfortunate gentlemen are told that the dead body lying in the middle of the room has a high enough concentration of toxins in its blood to be deadly. In fact the body isn't dead at all and belongs to the Jigsaw Killer, their captor
In the 1981 Clash of the Titans, Medusa's blood melts Perseus's shield - and not any ordinary shield, but one given to him by the gods.
In Living Hell the main characters immune blood is used more literally then in most cases, slicing his hands and then grabbing onto the viral tendrils. He also covers the female lead in it at one point, in order to offer her partial protection.
Pacific Rim: Kaiju blood is damn toxic, even after evaporating, and considering the sheer amount of blood a Kaiju has you can imagine the sort of cleanup hell that has to happen once one falls.
Similarly, the real danger from the monster in the classic kaiju film The Beast from 20,000 Fathoms is the prehistoric diseases in his blood that humans have no immunity to. The challenge is to kill him in a way that won't produce torrents of blood (they use a highly irradiated sniper bullet.)
In The Chronicles of Amber, Amberites can make Pattern Ghosts real with their blood. The blood of any descendant of Dworkin can also damage the Primal Pattern, which hold Amber and the Multiverse it spawns together. Bad Things Happen when an Amberite bleeds on that Pattern. The Chaosites, on the other hand, have blood that bursts into flame on contact with air. For Merlin, a Chaosite/Amberite hybrid, whether he bleeds fire or blood depends on whether the shadow reality he is in is nearer to Amber of Chaos.
There was a Weaponized Blood of Mass Destruction version in Charles Sheffield's last novel, Dark as Day. A character carries nanomachines in his bloodstream that, when dropped into a gas giant, will shift high-pressure hydrogen into a denser configuration, collapsing the planet and releasing a big enough burst of energy to destroy civilization.The character also has mental programming to seek out an opportunity to dive into a gas giant...
Witch's blood is deadly poison in the Ever World books, though I don't believe it ever gets used against people. They just hold a very important plant hostage.
In the Star WarsExpanded Universe, the voxyn, Jedi-hunting beasties created by the Yuuzhan Vong, have blood that becomes a deadly neurotoxin in most kinds of air... so even if you kill the thing without being bitten/clawed/spat acid on/hit with tail spines/eardrums burst by screech... you might still be screwed.
The scabmettlers of China Miéville's The Scar are a race whose blood clots extremely quickly. Before going into battle, they cut themselves in certain ritualistic pattern, and the results blood flows harden into armor and weapons. Blessed with Suck to some extent, since they need to medicate themselves constantly or risk spontaneous clots that will turn them into statues.
Bora Horza Gobuchul, the changeling protagonist of the Culture novel Consider Phlebas, can makes his own blood and saliva highly poisonous, so guess what happened when the leader of a cannibal sect tried to eat him
J. R. R. Tolkien's dragons have poisonous blood, corrosive and/or blazing hot; if you have the guts to sword a dragon's belly, chances are that you will be killed by it, or at least disfiguringly burned/scalded.
Granny Weatherwax from the Discworld novel Carpe Jugulum. There's really not more to it than that: she uses blood as a weapon, and the weapon is her. Well, the blood is a delivery system...
Myrddraal blood in The Wheel of Time series will corrode the metal if not cleaned. Other types of shadowspawn have the same effect, only faster so their blood needs to be cleaned immediately.
In the Barbara Hambly novel Those Who Hunt the Night, a vampire character kills a vampire by injecting himself with a lethal dose of silver nitrate (silver being fatal to vampires in this setting) and allowing the vampire to drain him before he succumbed to the toxic effects of the silver nitrate.
In the Doctor Who novel 'Forever Autumn' the Doctor temporarily disrupts the antagonists' biological ship by smearing a drop of his blood over one of its surfaces. Later on he threatens to overwhelm the ship completely by slitting his wrists all over the floor.
Duke Venalitor from the Warhammer 40,000Grey Knights novel Hammer of Daemons has the ability to make any shed blood into prehensile tendrils.
In The Sword of Truth, there is a scene where Richard kills some monster, and a drop of its blood on his arm requires later treatment. As it dies, the monster transforms into a mass of snakes. Richard kills a few snakes, and they transform into bugs, whose bites must also be treated.
Marshal Atkins, from John C. Wright's Golden Age, has the Blood Horrors type: intelligent nanites that can poison or corrode, as ordered.
The oldest of Anne Rice's vampires have the Boiling Blood variety, called the Killing Gift. This is essentially how Akasha kills most of the world's vampire population in Queen of the Damned; she ignited their highly flammable changed blood and set them on fire from the inside out. The power later got retconned into two separate abilities, with the Killing Gift not really being detailed but assumed to make ruptures in the victim's cardiovascular system.
In John Scalzi's Old Man's War novels, enhanced human soldiers have their natural blood replaced with SmartBlood, composed of nanobots. While not explicitly intended as a weapon, in The Ghost Brigades, Jared Dirac discovers that it is possible to remotely instruct the nanobots to explosively combust once outside the body. He uses this technique to momentarily stun an attacking alien after his tongue is cut during combat.
Dark Angel's second season featured a plot arc based on Max's DNA having been altered to be poisonous to Logan on contact. Not blood, exactly, but any secretions would have the same effect.
In Oz, Simon Adebisi steals blood from the AIDS ward, and uses it to infect Antonio Nappa via a hypodermic needle stick.
When the alien hybrids from The X-Files were injured, their green Alien Blood bubbled and released some kind of toxic gas or something. It was also both corrosive and lethally infectious. Putting the victim on ice could slow the effects, and a course of antiviral drugs could remove the infection.
When Angelus gets loose in Angel, Wes and Faith take him out by using Faith's blood against him. They know Angelus won't be able to resist trying to turn a Slayer, so they inject Faith with a dangerous hallucinogen just before the fight, then let him win. It's a big risk, but it works — Angelus is unconscious long enough for Willow to restore his soul.
In The Conditions of Great Detectives one episode was all about trying to discover what the weapon was, as there was absolutely clue as to what it could be. In the end, it was revealed that the murderer, who had access to blood bags at the hospital he worked at, froze some blood of the same blood type and stabbed the victim with it, which would eventually melt and get lost amongst the victim's blood. This was subverted when it turned out the victim had fallen off the roof and stabbed himself in the chest with his own shin bone.
Werewolf blood is toxic to vampires in Being Human. George exploits this in one episode by biting his own wrist and forcefeeding his blood to a vampire, killing it.
In the Haven episode "Bad Blood", when Mike Gallagher's blood is shed, it gains a life of its own. It starts out as a few drops, but it can drain a person's blood completely through skin contact and add the stolen blood to its own mass. Mike had no control over it.
One of the titular "Warriors of Time" in the music video for the song by Black Tide seems to have this power. He even creates a tidal wave of blood that floods into the enemy giant robot, making it explode.
Myths & Religion
In Greek Mythology Nessus was a centaur killed by Hercules for trying to rape his wife, but Nessus somehow convinced Hercules's wife that his blood was a love potion, so one day she smeared it on his shirt. Hercules was poisoned by the shirt and in agony from the non-fatal corrosion, immolated himself to end the pain. However, the centaur's blood itself wasn't the poison — the arrow used to kill Nessus was one of those that Hercules had poisoned with the blood of the Hydra of Lerna, one of the monsters that he slew during his Twelve Labors, and the hydra's blood was mixed with that of Nessus.
Older Than Dirt: In Enuma Elish, the Mesopotamian creation myth (ca. 1600 BCE, maybe), the evil mother goddess Tiamat gives birth to dragons that, among other awesome features, have "venom for blood". Bad-ass.
The Asura Raktavija could regenerate from a single drop of blood, therefore requiring Kali to drink all of his blood before he could be killed once and for all. In other versions, she strangled it.
Another myth included a monster who would spawn mooks from his blood whenever wounded. Unable to beat him, Durga transformed into Kali, who promptly solved the problem by eating him whole.
The dragon Fafnir of Norse Mythology had corrosive blood in some versions of the story, and this required Sigurd to dig a pit to catch the blood in order to avoid being killed by it. (In other versions, the blood was not corrosive and indeed conferred invulnerability on Sigurd when he bathed in it.) Some have a bit of both; though bathing in the blood made him invulnerable it felt like bathing in Hollywood Acid.
The European basilisk was a creature so vile, in addition to its venomous bite and habit of causing people to drop dead just by looking at it, also had blood that was so toxic, if a man stabbed it on horseback, the poison would radiate through his weapon, killing not only the man but his horse.
In some versions of the story, the blood falling from Medusa's severed head dripped down into the Sahara desert over which Perseus flew, transforming into venomous snakes. In other versions, Pegasus sprang from it, and in still more, her blood petrified Atlas and turned seaweed into red coral. Really, there wasn't a lot it didn't do.
In Mortasheen, the Viviphage has the power to control blood, ripping out the blood of its victims. It also happens to be the ultimate form of the Vampire race and look like a bacteriophage. There's also the Mothneaser, one of the game's Olympus Mons that produces massive quantities of blood that it doesn't just turn into weapons or horrible "blood golems" but also can insert tendrils of its own blood into people to puppeteer their bodies.
Maho-tsukai, or bloodwitches, who use their specific magic discipline is fueled by blood, sometimes their own (but usually those of unwilling victims), to accomplish all 5. They are ALWAYS evil, in this morally gray setting.
One particular type of Oni has flaming blood, and anyone who strikes the Oni with a sword must dodge to avoid being set on fire.
Being exposed to the blood of Tainted creatures almost always carries the risk of becoming Tainted.
It's frequently mentioned in Warhammer 40,000 that Space Marine blood is toxic, either made that way intentionally, or as a side effect of their many, many, MANY alterations to make them superhuman.
Hardly an unexpected trait for inhuman Super Soldiers who also secrete acid saliva and are capable of eating a healthy diet of, among other things, concrete and metal.
In an older edition of the Tyranid codex, some large Tyranid beasties could buy "acid blood" as an upgrade. When killed they would damage nearby units.
Though not poisonous blood per se, the Eversor asassins have their bodies pumped with so much stimulants and painkillers that when killed they will trigger a "bio meltdown", causing the asassin to explode and damage units in base contact.
Not to mention the Chaos Space Marines, who, in addition to being supersoldiers, worship gods that give mutations as gifts, and thus have some bad stuff in their blood. Specifically Noise Marines, whose blood is most likely about 50% combat stimms (many of which are probably not too healthy for a normal person to touch), or Plague Marines, whose blood is full of many, many horrific diseases.
In the previous edition of Warhammer Fantasy Roleplay, trolls were very dangerous to kill because of their stomach acid. Infamously voracious and able to digest everything short of magic items, they had oversized stomach filled with very potent acid. If one was killed with a critical hit to the torso, his belly would burst, dealing massive damage to anyone standing too close.
In the old siege rules dark elves poured boiling blood rather than boiling oil upon their opponents. While not as dangerous in damage potential it had psychological effects on the opponents.
The Blood Substitution Chaos mutation turns a Warhammer Fantasy Roleplay character's blood into another substance. Some of the possible substitutions (e.g. acid, molten metal, mice) can inflict damage on enemies when they wound the character. Apparently you can survive being the victim of this mutation because Chaos Did It.
In fact, in most of the Warhammer/40k games, there are aspects that allow this - either the aforementioned alien acid blood, exploding corpses due to vicious stimulant use, or (in the Fantasy game) chaos gifts that strike back at their attacker with a gush of ichor/blood/acid/all three. However, in the 1999 spin-off game Mordheim, one of the spells was named "Burning Blood" and literally consisted of slashing open a palm and showering an enemy with blood, which bursts into flame in mid-air. And then testing to see if you collapse from blood loss.
Needless to say, Vampire: The Masquerade had quite a few Disciplines that allowed the player to weaponize blood. The Assimite Discipline, Quietus, had a power that allowed the user to turn their blood into a deadly acid, and the Tremere Discipline of Blood Thaumaturgy allowed them to control another person's blood (up to and including boiling it). The Tzimisce, meanwhile, could turn themselves into blood, and at a higher level than that could make the blood in their veins acidic - while this made it impossible to create new vampires or ghouls, effectively rendering htem sterile, it also rendered diablerie (soul draining) next to impossible as well (which, given that a vampire that powerful was likely very, very old, was quite important to ensure).
And the Kuei-jin have Blood Shintai, essentially a grab-bag of all of the above, from making their own bodies constrict or swell, creating whips of blood, or turning others' blood into poison.
In Vampire: The Requiem, the Norvegi have a discipline that allows them to mod their own bodies and craft their crystallized blood and bones into blades or armour. At the highest level they cause blood in their bodies to erupt into spikes that shoot out in all directions, impaling anyone nearby. It is also noted to hurt the using vampire as hell (unless a critical success is obtained) and destroys their clothing.
Forgotten Realms spell "Blood Lightning" - remains indefinitely as a "curse", after bloodletting begins, the subject can choose whether to receive a weak healing or sweep everything else around with red lightnings discharged from wounds. Also, Chosen of Mystra tend to bleed "Silver Fire", a form of raw magic. In the fall of Myth Drannor a colonel of Army of Darkness caught overconfident Khelben (then "Nameless Chosen") and tried to tear him in half — he almost succeeded, but ended up completely obliterating himself and everyone nearby... while Khelben survived (barely).
In 3e and 3.5e, the Blood Magus prestige class is entirely built around blood-based abilities, including stepping into one creature with blood and out another, and granting temporary sentience to an enemy's blood, which then proceeds to attempt escape. Painfully.
3.5e Spell Compendium includes a spell that causes the victims veins to escape from the victim. Appropriately it requires a save against instant death and even if that succeeds the person takes massive damage and then must untangle their own entrails in order to escape.
Related to this is the 3.5 Assassin spell Heartripper, which, if the target fails a save vs death, blasts the target's heart out of their back.
D&D 4e features a defensive use in its magic Bloodcut Armor, with allows a sufficiently injured character to ignore a certain amount of further damage when it is active.
According to the Fiendish Codex: Tyrants of the Nine Hells, every drop of Asmodeus' blood that falls to the ground turns into a pit fiend.
Similarly, if the Stigmata Key from Geist The Sin Eaters doesn't deal with ghosts, then it deals with blood, usually in this sense. Some uses include the Stigmata Curse (which causes a person to bleed constantly from their fingernails for days), the Stigmata Caul (which allows a Sin-Eater to move their blood, detach their arm and remote control it, and create a miniature blood homunculus) and the Stigmata Rage (which not only hurts someone with pure ghostly force, but makes sure their wounds won't close easily).
In addition to being Cast from Hit Points, the Whateley clan's Blood Magic in Deadlands has a number of spells that bolster the caster's abilities. For instance, in exchange for letting dark powers devour some of the Blood Mage's tasty, tasty essence, the spellslinger might get Instant Armor. In exchange for actually doing their bodies overt physical harm, they might recover some of the game's version of Mana. (In case you're not getting the idea, that last ability is called "Faustian Deal")
The Abyssal Exalted are able to extrude chains of their own blood to use as weapons or just tentacles. On a side note, one of their martial arts specializes in whips, chains and bondage. Yes, they're compatible.
One set of Green Sun Prince charms allow them to control liquid. This include spilled blood, and there are tons of it on the battlefield. A shoggoth made of blood is one of the nastier things they can sic at their enemies.
KAEDE Smith in Killer7 has the special move "Blood Shower" during which she cuts her own wrists spraying blood everywhere and unveiling any hidden passages.
One of the Hydra units in Heroes of Might and Magic 5 has acid blood, allowing them to harm any creature that attacks them in melee.
The Metal Slug series has a zombie apocalypse stage in some of its iterations. When you get hit by a zombie you don't die immediately, but you become a lumbering zombie yourself. Your gun as a zombie is crap but your grenades become a stream of blood that your character vomits in an arc that sweeps over half of the screen.
In Turok 2 there were zombie enemies that could throw blood at you. If you went to the options screen and turned the blood off, this section became a lot easier.
The tool of the trade for Blood Assassins in Dungeon Siege II: Broken World. They have both the Bloodsoaked Shots passive ability (which increases damage at the cost of health) and the Ravaging Strike ability, which causes a large amount of damage that drains 40% of the user's health, and with the correct upgrade, it can set the target on fire.
The Big Bad of Akiha's path in Tsukihime can turn his blood into swords. Even at a distance. Even when it's soaked into your shirt.
Jedah Dohma from Darkstalkers fits into this. All of his EX attacks involve turning his blood into a mass of giant hands to grab and abuse his opponent. Or he can rip his head off and spray his enemies with the blood from his wound.
The Carpathian Dragons from BloodRayne 2 are handguns which use blood instead of bullets. Rayne can refill them by stabbing them into enemies, or use her own blood in a pinch.
Ikumi Amasawa of Eternal Fighter Zero uses her own blood as well as the blood of her victims for her special attacks, such as plunging her own blood into the ground to call forth an erupting fountain of blood.
Guild Wars has Necromancers, which have an entire skillset (blood magic) devoted to this. And even many of the skills outside of blood magic still require sacrifice of life points to operate.
From Pokémon Ruby and Sapphire onwards, there's the Liquid Ooze ability. Any Pokémon that uses a health-draining attack on a Pokémon with this ability will take damage instead of regaining health. (Dream Eater works normally, as it's a psychic life force draining attack, not a biological blood-sucking one.)
Alexia Ashford from Resident Evil: Code: Veronica flings her blood at the player throughout the first boss battle with her. When exposed to the air, her blood catches fire.
She also indicates this is a dangerous technique, since if you let her use too much she dies at the end.
In The King of Fighters, Lin, a Chinese ninja, has his blood type listed as "poison". In gameplay terms, this means that some of his moves involve momentarily covering his hands with his blood in a way that makes it look like he's wearing surgeon's gloves made out of blood, and he has a super move that poisons the opponent. Turning the blood off from options turns it green instead, and makes his winning portrait look like his hands are covered in snot.
The Warcraft 3 custom map Tides of Blood has a hero known as the Blood Mage, which is different from the standard Blood Mage in that his title is a bit more literal. He can use his own HP to summon geysers of blood and "Blood Elementals". His ultimate technique is, naturally, Tides of Blood.
In World of Warcraft death knights get Blood Horrors, Boiling Blood, and Blood Boost. Though they gain power from the spilling of their enemies' blood as well as their own.
Deathbringer Saurfang, strongest Death Knight of the Lich King, is a Blood death knight turned up to eleven.
Deathwing bleeds Old God-corrupted magma which is highly corrosive and spawns Horrors.
Nespirah's blood cells are big enough to absorb most humanoids. It means no harm, but only has limited control over it's bodily functions.
Hakkar's Corrupted Blood was not only highly toxic, but infectious enough to turn into a full-fledged pandemic.
Testament from Guilty Gear has a Scythe made out of his own blood. His sprite shows that there's a hole in his hand that he draws the blood out of.
And one of his special moves involves cutting himself and forming a web out of the blood spray, to trap his opponent.
The Mainliners from The Suffering are, essentially, the embodiments of death by lethal injection, so they don't have any blood in the proper sense, instead leaking a greenish fluid that burns you if you step in it. They indirectly use this as an attack as well—the same fluid is contained in syringes that stick out of their backs like porcupine quills, which they either throw from a distance or stab you with For Massive Damage.
This is one of 0's attacks in Kirby's Dream Land 3. Yes, you read that correctly. The True Final Boss of a K-A rated SNES game cuts itself so its blood will attack Kirby...and also rips out its own bloody iris as a last-ditch attempt to kill Kirby.
And then cue Zero Two in Kirby 64: The Crystal Shards. He friggin' cries blood, to be expected since he's more-or-less a reincarnation of Zero; moreover, the tips of his wings are soaked with the stuff, and may shoot blood at Kirby, too. (Even though it looks more like green gas; this could just be a form of censorship.)
It also splatters about whenever you hit Zero-Two in its eye, and flies everywhere once the normal Zero is defeated. However, due to the graphics it can be mistaken for red energy or explosions.
Lord Erebus of Demigod has an upgrade that turns his blood to poison, increasing his health regeneration and dropping a poison potion on death that damages, rather than heals the collector.
Geralt The Witcher can learn a recipe for a potion that turns his blood into poison. Particularly effective against vampires and other blood-drinking monsters.
In Dragon Age: Origins, the Blood Mage specialization allows a mage to use his own blood (as represented by HP) to cast spells instead of mana. They can also boil their enemies' blood in their veins, paralyzing and injuring them.
Or any mage can gain the Power of Blood ability to drain their hit points to replenish their magic rather bloodily if they pick the Dark Sustenance ability from the Warden's Keep expansion.
One of the enemies in Bio-Hazard Battle is a heart with spider legs. It shoots out blood cells that home in on you.
Remilia Scarlet of the Touhou series is another vampire capable of weaponizing blood. As Marisa said in her Grimoire, "Blood is amazing".
In the Age of Mythology Titans expansion, there are the Lampades nymphs (Hecate's myth unit). When they are killed, they explode in a spray of yellow-green blood. Normally, it's not harmful, but if you shell out the resources for the right improvement...
League of Legends has Vladimir, the Crimson Reaper, who is described as a "Hemomancer". He can drain life, throw deadly streams of blood, temporarily melt into a pool of blood, and infect enemies with a deadly "Hemoplague", which weakens and damages them.
The new version of Dwarf Fortress includes randomly generated "forgotten beasts" (and other similar creatures) which occasionally have poisonous blood. Dwarves can't be bothered to clean up blood, instead tracking it all over the place and causing the entire fortress to be infected. Hilarity Ensues.
There are also subterranean creature called "blood man": human shaped and sized being made entirely of blood. They're far less intimidating than they sound though, as being made of liquid makes them too soft to damage anything and they're torn apart by pretty much any attack.
The flash game Medieval Rampage 2 demonstrates this in its most basic form—kill various types of monsters, and they leave behind various types of unpleasant goop that you're better off not stepping in. Initially, you just encounter black goop that somehow blinds you and orange goop that slows you down, but green blood (identified as "acid" in the achievement list) actually damages you if you touch it. The overall effect is to constrain your movements as the waves of enemies get thicker and thicker.
The main bosses of Grandia II are the parts of Valmar (read: Satan) - individual representations of a demonic entity's anatomy. Valmar's Heart is just that: a giant, floating heart which spews blood from its open arteries.
Sacriers from Dofus can use thier blood to drag enemies towards them.
Batman: Arkham City has The Joker inject Batman with his own, TITAN infected blood, which would have killed him in under ten hours, had it gone untreated.
The Berserker subclass of Dungeon Fighter Online has several skills that inflict the bleeding status. The ultimate attack of the Hellbringer, an awakened Berserker, involves summoning a sword made of his own blood.
Mortal Kombat 2011 DLC character, Scarlet, uses her own blood as a weapon and is, herself, a creature made of blood. Noob Saibot in the same game bleeds a black inky substance that he also uses to create the clones that he uses in his attacks.
DLC character Eliza is a straight example of this, molding her blood into various Egyptian shapes to attack, and her proposed playstyle is that when she gets hit, she can control the blood that spills on the ground as a result for more special moves.
Diablo II's Necromancer has the Blood Golem skill as part of his Summoning skilltree.
Charlotte from Rumble Pack uses blood for some attacks. Since she is a vampire, it is unclear whether the blood is her own or comes from her victims.
In general, if an enemy in The Binding of Isaac has a ranged attack that dissipates on contact, but doesn't explode, it attacks by spraying blood. Some upgrade items make Isaac's blood deadly as well (mostly ones associated with demons, like the Book of Belial, but also one holy item, the Crown of the Martyr.)
Feral Ghoul Reavers in Fallout 3 (only appearing when Broken Steel is installed) throw gobs of explosive, radioactive gore as a ranged attack.
The final boss of Alisia Dragoon fires blood 'bullets' from his exposed heart at the heroine.
In Disgaea 4, one of the succubus' magichange attacks has her strip in front of the party member that was wielding her, resulting in a massive Nosebleed that damages, if not outright kills enemies in front of said party member.
The Secret World has Blood Magic, which is good for boiling enemies' blood, attacking them with spikes of congealed blood... and healing and shielding your allies.
Soul Sacrifice has one type of spell which allows you to fire a spray of blood, dealing damage at the cost of constantly draining your HP while firing. It's the only spell which can be used without limitation (Besides running out of health).
Despite the name suggesting Urine Trouble, the Golden Shower weapon from Terraria actually fires a yellow stream of "ichor", which is basically highly corrosive demon blood.
Carmine of Under Night In-Birth wields his own blood as a weapon (as well as hair dye), whether crystalized (normal attacks) or liquid (special attacks, which are usually Cast from Hit Points and leave temporary blood pools to empower further attacks).
The dragon in Dra Koi turns her blood acidic in order to fight the hero that much more effectively.
The Secreted Weapons of Doyt Gyo in Schlock Mercenary. Not limited to fluids; his... solid products are explosive.
Zebra Girl: All of Sandra's bodily fluids are highly acidic.
Mr. Jinx in Starslip Crisis, among numerous other Bizarre Alien Biology traits, has corrosive blood that can actually be ejected from his eyes, and as another character unpleasantly discovers, his blood is alkaline, not acid.
"I don't think you understand the difficulties I have. fire proof eyepatch, fire proof gloves, fire proof clothes... Just to keep them from melting whenever I get a scratch."
This is the special ability of Noblesse's main character, Cadis Etrama Di Raizel, as seen hereandhere.
Forget mere acid blood. The character codenamed Tennyo in the Whateley Universe has anti-matter (or something scientists cannot distinguish from anti-matter) in her cells, so she can release anti-matter into her blood when she wants to. It doesn't ever harm her, but you don't want to be hit by her blood if she gets injured. (Note that this doesn't usually seem to be a major issue because she heals just that damn fast — but there is one canon scene where she deliberately bites her lip to shatter a block of solid ice she and a teammate are encased in because she's worried about just cutting loose with her energy blasts. It works.)
While on the subject of Whateley, in Hawthorne Cottage (the one for those students who really can't easily lead a normal life even for mutants) there lives a girl who can't ever leave her room because she's hooked up to life support and her blood is so incredibly toxic that visitors or cleaning personnel have to wear sealed suits and follow strict biohazard protocols. Compared to her, many of the other kids at Hawthorne (to say nothing of other students who have trouble adjusting to their mutation-induced changes) have it easy.
Survival of the Fittest: Evolution's Taryn Gregory has her blood crystallize into sharp shards under exposure to oxygen, which can be broken off from the wound and used as a weapon.
A throwaway gag in Family Guy involved showing a video on how to identify gay people. One indicator was ownership of a Madonna album; the other was having "deadly corrosive acid instead of blood."
An advanced Waterbending technique in Avatar The Last Air Bender is Bloodbending, harnessing the water in an opponent's blood and controlling it in order to turn them into a living puppet.
Given how Energon is both used as Symbolic Blood and the standard fuel sources for both all Cybertronians and their various guns, one can consider Optimus Prime's Energon axe to be made out of his own "Blood" formed into a laser Axe.
There are many criminal cases stemming from a perpetrator with a blood-borne diseases purposely having intercourse with, bleeding on, or stabbing victims with used needles.
In 1994, a woman named Gloria Ramirez suffering from late stage cervical cancer was brought to a hospital in California. When one of the nurses treating her tried to draw blood, she noticed a strong smell of ammonia. One of the doctors then noticed small particles in the blood. A few minutes later, the nurse fainted, and shortly after that, the doctor got sick to her stomach and fainted herself. A few minutes after that, somebody else fainted. Months later, Livermore Laboratories concluded that the woman had been taking dimethyl sulfoxide for pain, which had been building up in her bloodstream due to a urinary blockage and that her treatment in the emergency room caused it to turn into dimethyl sulfate, a poisonous gas.
Eel blood is toxic.
Supposedly, the Lyme Disease vaccine LYMErix can grant this. In addition to preventing the disease, it kills the offending tick through the vaccine carried in the consumed blood. Somehow...