Blood is a powerful symbolism, and sometimes even has mystical powers. The Blood Oath makes use of this to make a commitment that can't be broken. (Or if it is, it's crossing the Moral Event Horizon)
Bob has a secret he needs to get off his chest, but doesn't want it to be made public. Alternatively, he needs to be certain Al will do something for him. Either way, this is so serious that it requires making a blood oath. After all, once you're bound by blood, you can't break that promise, even if you're a villain.
In one common version, Al and Bob become Blood Brothers, cutting their hands open and shaking on the deal. The implication here is that brothers would NEVER betray each other
In another, Al cuts his hand open, and simply lets the blood drip to the ground. This is sometimes highlighted by the phrasing of the oath: "If I break my word may the Earth drink my blood!" Then giving the Earth a 'taste' of your blood so that it knows what to look for if you fail to keep your oath. Sometimes this version is used to swear an oath of vengeance- and making it clear that you mean business.
If it uses blood and binds you to an oath, then it belongs here.
Can overlap with Couldn't Find a Pen, which is any kind of writing in blood. Contrast with Heroic Vow. See also Geas. May or may not involve actual Blood Magic. When someone has broken this vow so thoroughly they become ostracised they're The Oath-Breaker.
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Anime and Manga
It's implied that this is the traditional way in Mahou Sensei Negima! to induce a Pactio. But that's messy and doesn't have nearly enough fanservice, so they kiss instead.
This is also all one really has to do to exchange mana in the Nasu Verse as well. Again, sex is more fanservice.
In Naruto, an interesting variation occurs as Naruto swears to Neji on Hinata's blood during the Chunin exam preliminary battles for the final test, "You... are going down", promising to defeat him. He makes the same gesture at the beginning of their fight in the finals.
Summoning contracts are also typically sealed in blood, with a small blood offering made each time the summon is invoked.
In Mobile Suit Zeta Gundam, Paptimus Scirroco seals his loyalty to the Admiral Jamitov Hymen and Titans with a bloody thumbprint. This does not stop him from murdering Jamitov and taking over the organization.
A Growing Affection has the Blood Oath Jutsu, the user swears an oath, sets a condition for breaking said oath (two examples in the story are memory loss and death), and then writes the kanji for Honor over his or her heart in his or her blood. Breaking the oath will cause the condition to occur. Only one person in the world has ever successfully removed a Blood Oath seal, and she has less than a fifty percent success rate in doing so.
Kevin Costner's Robin Hood does it when he swears to avenge his father's death.
The Outlaw Josey Wales and Ten Bears take a blood oath to seal the "words of iron" peace treaty between the Comanches and Josey's friends at the Turner Ranch and Santo Rio. It is strongly implied, though unstated, that this also makes Josey and Ten Bears Blood Brothers.
It is revealed that before the events of the Gaunt's Ghosts series, Gaunt and several other officers made a blood oath with Warmaster Slaydo, promising to continue the Crusade and protect Saint Sabbat to the best of their efforts in the event that he died (which he did), and that this is the reason Gaunt is willing to disobey Lugo's orders.
Marie Brennan's Doppelgänger duology features a blood-oath as a plot point in both books. The blood is technically used as a focus just to shape the spell, but breaking the oath does involve bleeding to death through the one wrist cut while swearing the oath, so it still counts.
In Tamora Pierce's Tortall Universe, anyone who breaks an agreement that they signed with their own blood will die as their blood literally boils in their veins.
The Bazhir of Song of the Lioness have a lot of blood oaths. Members share blood to induct new people into the tribe, as part of the ceremony of making someone into the Voice of the Tribes, and as an adoption ritual. Alanna, who becomes part of the Bloody Hawk tribe, once sighs, looking at the scars she's already picked up while with the tribe and thinking she's just vain enough to not want to pick up more.
Used frequently by Aly in Daughter of the Lioness when she's turning enemy spies into double agents for the rebels—it's harsh, but it discourages betrayal better than milder options would, so she doesn't let herself be bothered by it.
In The Baroque Cycle, the members of the Cabal swear various oaths of their loyalty to each other and specifically to Jack. Van Hoek swears by his right hand. When Jack reminds him "But Cap'n, you are left-handed!", van Hoek points out that he'll need his strong left hand to chop his right off if he ever breaks the oath — and then, as a symbol, he chops his own little finger off and throws it into the fire. (He then faints into the dust, but it's the symbolism that counts with these things.)
In the Incarnations of Immortality series, when 2 immortal Incarnations sign an agreement, they choose to shed some blood to sign their names. This binds even the Incarnations to the deal. It happens once between two Incarnations Fate and Satan.
Live Action Television
In Farscape, when John needs Scorpius's help to rescue Aeryn, Scorpius first makes him perform a "Scarran blood vow", which involves both of them cutting their fingers and drinking each others' blood and then their own. John is understandably squicked.
In the last season of Angel, Angel signs a contract from Wolfram and Hart in his own blood. He learns this when the person offering him the pen stabs it into his hand.
Lost in Space episode "The Space Pirate". After the title character takes Will Smith hostage, he decides that he has to tie Will up before going to sleep so Will can't escape. When Will objects, he proposes that Will take the Pirate's Oath instead. This involves pricking a finger on each of their hands, putting their bloody fingers together and Will taking the Oath.
During a famous feud with Jerry Lawler in Memphis, legendarily crazy lumberjack wrestler Jos LeDuc famously took a blood oath (pronounced "oat") swearing he would end Jerry Lawler's career. LeDuc ramped up the squick factor several notches by using a double-bladed axe and REALLY CUTTING A SCAR INTO HIS OWN ARM!
In Earthdawn, magical blood charms are used to seal bargains: if either person violates the conditions, the magic punishes the violator.
Vampire: The Requiem has the Invictus (a covenant of fedualist vampires heavy on inherited power) and their Blood Oaths, magical bonds with heavy penalties for violation.
Changeling: The Lost has many forms of oaths, and your health or life can be put on the line for them. With heavy costs comes heavy rewards - the more you'll lose if you break the oath, the more benefit you can gain from it.
In Goethe's Faust, the title character's contract with Mephistopheles is signed with a drop of blood.
'Mephistopheles: Du unterzeichnest dich mit einem Tröpfchen Blut. (You sign it with a drop of blood.)
A quest chain in World of Warcraft starts off with you making one of these with a troll, to show that you'll both honor a truce. He honors it, in that he never does anything to harm you, specifically, although you're tricked into doing horrible things.
In the backstory, the original Orc chieftains on Draenor gave themselves up willingly to the Burning Legion for power. To seal the pact, the chieftains drank the blood of Mannoroth, binding the entire Orc race to him while infusing them with his fell power.
In Fire Emblem: Radiant Dawn, a blood pact is a contract that two or more people sign in blood. If the terms of the pact are not upheld, the person's subjects die off—first slowly, then progressively faster hence the plague that struck years ago. They can only be broken by killing at least one person involved in the pact AND destroying the document.
Okku from Neverwinter Nights 2: Mask of the Betrayer swears in blood to stop the recently emerged spirit-eater (that is, you, the protagonist). If spared, he calls a truce which lasts as long as you agree to suppress your urge to devour spirits and breaks if you meet certain conditions (commiting some certain evil acts in Okku's presence or getting his approval below zero).
"Ares! Destroy my enemies and my life is yours!"Ascension reveals that this was a Blood Oath. Kratos spilled the blood of his enemies, the blood of the innocent, and the blood of his family. When Kratos turned against Ares after that last one, the Furies hounded him to bring him back to Ares' service.
Madoc of Wake the Sleepers accepts a contract through blood on a bounty, as his crazy Assassin culture requires.
Rowasu of Juathuur is full of scars because of this trope. He still breaks the last oath he makes.
In The Order of the Stick, a Blood Oath binds not just the oath-taker, but all of their descendants. And if they don't die trying to fulfill the oath, they're denied entry into the afterlife while their children have a go at it.
Of course, the specific instance mentioned here (namely, Eugene Greenhilt's oath against Xykon filtering down to his son Roy) is an odd case. The father in question abandoned the oath pretty quickly, only to try to force it on his child when he was getting old. After the child's death (he died trying to fulfil it), he gets into Heaven while the father is stuck in the "waiting room". When the father gets enraged at this, the angel who made the decree chews out the father for being so careless with a blood oath and points out that the child didn't actually have to take up the oath, but doing so out of free will and trying his best to fulfil it makes him a better person than the father.
It should also be noted that "Start of Darkness" depicts taking the Oath much like getting a tattoo. Including the part where Eugene got it while he was smashed and had little recollection of doing so later.
In Bad Machinery Erin Winters demands that contracts acknowledging people owe her a favor be signed in blood. Given that the last time she was seen in Scary Go Round before reappearing without explanation in Bad Machinery she was in the company of and (technically) married to Aleister Crowley, and trapped in Hell, this may bode ill.