Maybe pen and paper weren't available. Maybe that just wouldn't be cool/creepy/dramatic/funny enough. This is a trope for writing with unusual equipment.
A particularly common (and gruesome) version is a victim of a murder or monster attack to use the last of their blood and strength to write a Dying Clue. Also the blood will often be immune to oxidization, remaining bright red no matter how long.
See Also: Distress Call, Room Full of Crazy and Deal with the Devil; all of which use or overlap with this trope to some extent. There may be Bloody Handprints nearby as well.
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Anime & Manga
Bleach uses an instance that plays this trope every way simultaneously except for straight. Urahara tells Ichigo to leave his window open on the night that they leave for Soul Society, and on that night, he sends a balloon through the window, which splatters over Ichigo's wall. The liquid in the ball (which looks alarmingly like blood to Ichigo) drips down and forms a message. After the important bit of the message has been formed, it keeps on going to form a post-script: "Anyone who thinks this looks like the message of a dying man has no sense of humor."
Not even five minutes later, Ichigo runs into Chad and Orihime, who mutter about the fact that "apparently [they] have no sense of humor," leaving Ichigo to think "So they got that message too..."
In Death Note Light keeps a piece of death note paper and a pin for pricking his finger and writing in blood (couldn't fit a pen?) concealed in a secret compartment of his watch, so that he can kill someone quickly and discretely if necessary. He first uses it to kill the third Kira when he becomes a liability rather than an asset, while L is sitting right next to him. In the final episode after getting his pen destroyed by Matsuda while trying to write down Near's name in the pages of the titular Artifact of Doom, Light continues writing with his own blood, which prompts Matsuda to blast the living snot out of him. This is actually a pretty severe "What the hell were you thinking" moment, as apparently at this point he had lost all self-control whatsoever and couldn't just calmly write down the name.
Parodied in one of the omake strips in the back of one of the Fullmetal Alchemist volumes — Roy reveals that he wants to make all the women in the military wear tiny miniskirts, but then also admits that he'd fire all of the men at the same time, which prompts them to shoot him. His last words are written in his blood on the floor next to him: "Miniski..."
Lest we forget, Al's blood seal? The iron in the blood made it possible to bind his soul to the armor, so even if Ed could have found a ballpoint while flailing around in a sea of his own blood, it wouldn't have worked quite as well...
Also the scene where, after losing his gloves and the runes inscribed on them, Roy carves his alchemical runes into his own skin.
Ghost in the Shell. "Puppeteer'' is written in blood by the General in "Solid State Society" after he's forced to commit suicide by the hacker.
In Hellsing, a pair of vampiric serial killers write blasphemous messages on the walls of their victims' homes using the victims' own blood.
During their first encounter with Alexander Anderson when Alucard gets pureed, his sentient blood writes a message to Seras all by itself.
Parodied by Sayo's chapter in Mahou Sensei Negima!. Trying to calm down the panicking class after they mistook her for a malevolent ghost, Sayo wrote "It's a misunderstanding" on a window. Unfotunately, since being a ghost, the only writing material she had was blood, and "It's a misunderstanding" in katakana is the same as "Death five times", all it did was make the class panic more.
In chapter 53 of Mirai Nikki after the second Yuno is stabbed by the real Yuno, the other Yuno uses her last moments of life to write "Help Me" on the wall in her blood.
In Naruto: Jiraiya etches a dying message on a toad's back.
In Princess Tutu, Drosselmeyer is said to have written a story in his own blood after the Book Men cut off his hands. Yes, it's implied he used the stumps.
Parodied in Soul Eater. During an episode involving the entire cast taking a test, Black*Star gets caught trying to steal the answers. The teacher beats the crap out of him and hangs him on the board as a warning. Later, when Soul is getting very desperate (having had his cheat sheets taken away), he sees Black*Star writing on the board in his own blood. Cue melodramatic inner monologue about how he must be trying to save his friend in dire straits- until Soul realizes it's an autograph. He is not amused. The audience is.
In Mobile Suit Zeta GundamEnigmatic Minion Paptimus Scirocco signs a written pledge of his loyalty to Big Bad Jamitov Hyman with his own blood. Naturally, this means he eventually betrays Jamitov and usurps his position as leader of the Titans.
In "Calliope", The Sandman has a characteristically squicky example when Dream curses a writer with ideas. Lots of ideas. All at once. Making him try to scratch them out on the walls with his fingertips. It's hard to feel sorry for him, though, considering he kidnapped and imprisoned the titular muse of poetry just to get ideas for stories and get rich. He also rapes her from time to time, too.
In an early Batman comic, when Linda Page is being kidnapped, she insists the kidnappers give her a moment to fix her make-up (Hey! It was the 1940s. Criminals were more polite) and writes a note for Batman in lipstick on her vanity table.
Played for laughs in Deadpool, when the title Anti-Hero meets up with Alex "Agent X" Hayden, wins their scuffle, then proceeds to write messages using his entrails. And steals Hayden's pancreas For the Lulz. Don't worry, Hayden can heal.
Done literally in the Squadron Supreme trade paperback; the first edition was printed with the cremated ashes of writer Mark Gruenwald mixed in the ink.
A writer character in Shade, the Changing Man has the ability to extract the abilities and characteristics of real people for use in his stories. When he bases one character on Lenny he takes away her unique and caustic wit, and when she realizes this she freaks out by scribbling "It just isn't funny anymore" in lipstick on the bathroom mirror before trying to kill herself.
In One Hundred Bullets, a woman writes "He's going to kill me" in her lipstick on the bathroom mirror in Wylie's gas station, referring to her husband, who she's with. (Wylie acknowledges that he received the message when he compliments the woman on her shade of lipstick.)
In one MAD "A Mad Look At..." a dead man, having been fatally shot, used his blood to not only identify the killer (his partner), and also his motive for doing so. This is especially surprising in that "A Mad Look At..." typically has no dialogue, only RebusBubbles and small signs.
Done by a dying man in Constantine, using a corkscrew as a pen and his own hand as paper.
Who Framed Roger Rabbit? had two versions of this trope using the same sheet of paper! Roger's alibi for murder is that he was writing a letter to his wife with her lipstick on a "nice, clean, sheet of paper" which turns out to be Marvin Acme's will, (the MacGuffin of the movie,) which has been written in ACME's Disappearing-Reappearing Ink.
The Untouchables: After murdering Oscar Wallace and George the Bookkeeper in an elevator, Frank Nitti uses their blood to write the word "TOUCHABLE" on the wall.
In Quills, the Marquis De Sade has his writing equipment confiscated and resorts to writing on his clothes, first using red wine and eventually bodily fluids.
Averted in Memento where the main character is stuck without a pen and desperately needs to write something, but doesn't manage to find a workable substitute. However he does tattoo himself as a way to remind himself of important information.
Miss Froy in The Lady Vanishes writes her name with condensation on the train window. When the heroine sees it again, it confirms to her that she's not crazy and something is amiss.
In Transformers: Revenge of the Fallen, Sam starts having a mental breakdown in the middle of a frat party, and in order to get the symbols out of his mind, he starts drawing them on the table... with cake frosting.
Journey to the Center of the Earth (1959). Explorer Andre Sarcassan leaves a message in blood on a plumb bob which — encased in a lump of volcanic rock — is retrieved centuries later by the protagonists, sending them on their journey into the Hollow Earth.
In Charade, a knot of conspirators are dying off one by one - James Coburn's character is found with hands and feet bound to furniture legs and a plastic bag over his head - he used his finger to spell his killer's name in the carpet.
In Sisters, the dying man crawls to an open window, and writes "HELP" with his bloody finger, one of the few practical examples of writing something in blood in all of filmdom, as someone does actually see him do it— and there was a possibility at that point that he might not have died if he'd gotten the attention of someone a little more quick-thinking. By the time she figures out which apartment he's in, someone has managed to finish him off, hide the body, and clean up.
Megamind has the titular character writing notes about his plan with a jelly donut.
Played for Laughs in the Hong Kong comedy Justice My Foot.Stephen Chow plays the protagonist Sung Sai Kit, a cunning lawyer who wins via clever ploys and his wits (seriously, he wouldn't be out of place in the Ace Attorney universe). Jailed towards the end of the movie, he has to get the attention of the high magistrate, and as he is in prison, has no access to brush and ink. He writes a message in blood on a fan... by biting the finger of the man who presents the fan to him as evidence of innocence in a previous incident, and using said finger as a brush instead of biting himself, as is expected by Asian sensibilities.
In Moving Pictures, CMOT Dibbler tries to write down an idea for a film that came to him in a dream on his bedsheets; he runs out of bedsheet and starts writing on a wall, which he then pays a troll to carry around for him.
The same thing happens in Thief of Time, when Jeremy gets the idea for the Glass Clock in a dream and ends up writing the specifications for it all over his bedsheets and part of the wall.
In Men at Arms, Detritus, a troll, is trapped in a freezer, and the cold temperature brings his brain to peak efficiency. Unfortunately, this also means he's probably going to freeze to death, so as he gets smarter (and closer to death), he starts writing a mathematical grand unified theory of everything in the frost on the walls. He gets up to the = sign when he freezes up completely; when he's rescued, the heat from opening the door causes the rest of the equation to melt away.
In Thud!, a dying dwarf miner uses the last of his strength to scrawl a cursed mine symbol onto the door he was trapped behind.
Older Than Print: Writing an important letter in blood (complete with dramatic biting of the finger) is a great way to get across that your message is Top Priority. (That might be why in China, the Emperor gets to use red ink.) One such message is written in Romance of the Three Kingdoms in an attempt to get rid of Cao Cao; the note is found, and all of the signatories end up being fugitives of the government.
A mention of this trope is made in The Belgariad: When the party finds themselves in such a situation, Polgara has a quill, ink, parchment, etc, and explains that on a past occasion, she found herself needing to leave a note without the necessary implements, and ended up using her own blood to write the message. Following the event, she took steps to make sure it wouldn't happen again.
This also creates some Fridge Logic as this is a series where they can create anything at will. So if they needed a pen/paper/ink, they can just create them out of thin air.
Subverted in the Malloreon where Belgarath curses whoever was stupid enough to write an important prophecy on human skin: The ink didn't "hold" and Belgarath was forced to go yet elsewhere to try and find that prophecy.
Subverted in the Agatha Christie novel Death On The Nile: a murdered woman uses her own blood to trace a letter on the wall, presumably the first of her assassin's name. It later turns out that the killer wrote the letter. In a double-twist on this trope, it was written to implicate one of the people actually directly involved in the murder, in an attempt to make it look like another party was trying to frame her.
Mark Twain's The Adventures of Tom Sawyer. Tom and Huck Finn swear an oath to not talk about seeing Injun Joe murder Dr. Robinson. They write the oath on a shingle and sign it in their own blood.
Played with in Harry Potter And The Order Of The Phoenix. Harry is forced to do lines as punishment for telling the truth about Voldemort and is given a pen with no ink. He starts writing, and the message comes out carved across the back of his hand. Played straight in Chamber. Ginny writes threatening messages in blood while brainwashed.
In Mr. Bean's Scrapbook, the in-universe tie-in for the film Bean, a running gag is that Mr Bean keeps having to switch to new ways of writing: first his typewriter breaks, then his "borrowed" word processor is taken back, then he resorts to using a child's printing kit which takes three hours to lay a (mostly backward) sentence, then a pen which runs out, a crayon and a pencil which break, and finally he ends up using his own blood to refill the pen.
In Something Wicked, a murder mystery based on Macbeth, Duncan is found dead with "Malcolm" written on the wall of his tent in blood, which seems to implicate his son... except the hero realizes that everyone, including Duncan, called his son Mal, and it's unlikely that a dying man would have bothered with the extra letters. It turns out to be a frame-up by the real killer.
The Neil Gaiman-authored Sherlock Holmes/Cthulhu Mythos pastiche A Study in Emerald has the two main detectives (not Watson and Holmes) called to the murder of the nephew of Queen Victoria (actually one of the Great Old Ones). They find the word "Rache" written on the wall in blood. Green blood.
In A Study in Scarlet, Holmes finds 'Rache' written on the wall in blood. Lestrade wrongly concludes that a woman called Rachel is involved, and the victim died before finishing the name. Rache is German for revenge, and the murderer who was American intended for the police to believe it was committed by a German. Holmes wasn't fooled when he noticed that too much effort went into making the handwriting look German. The blood is also not from the victim, but from the murderer who's nose bleed because of the excitement.
In the Ellery Queen novel The Scarlet Letters, a dying man uses his own blood to write XY on a wall in an extremely cryptic Dying Clue.
And in the unrelated Vampire The Masquerade story "Scarlet Letters," a girl cuts her own throat and uses the runoff to start writing poetry on the wall while she's bleeding to death.
The Banjo Paterson poem "Clancy of the Overflow" contains this line (known to all Australian school children):
"And an answer came directed in a writing unexpected
(And I think the same was written with a thumb-nail dipped in tar)"
Stephen King's IT has Stanley Uris, one of the protagonists, commit suicide in his bathtub because he doesn't want to go back to Derry to face It again. When his wife finds him, she finds that he's written the single word "IT" on the bathroom wall in his own blood.
In Larry Niven's The Patchwork Girl the victim leaves "NAKF" written in his own blood on the rocks of the lunar surface. He was trying to write "NAKED" indicating that his killer was naked: i.e. not wearing a space suit, which is quite a trick out on the surface of the moon.
In Jonathan Strange & Mr Norrell Childermass briefly attempts to copy the Book of the Raven King, which had been written as blue discoloration on a man's skin, onto his own flesh. He quickly gives up for some of the many, many reasons no one should ever even try to copy out something in a language they don't know onto their skin, with a penknife, all alone in a cold and isolated area, when the book covered the man's entire body except for groin, face and hands, and for all he knows size and placement of the marks is vital.
In New Moan (a parody of the Twilight saga), this trope is played with. Teddy (the Edward Expy) gives Heffa (the Bella Expy) a note, written in some red substance. Heffa asks if it's blood, to which Teddy replies that blood is useless as ink since it clots too quickly, and it's just normal red ink.
In A Song of Ice and Fire people who join the Second Sons traditionally sign in blood. They abandoned this tradition some time ago and started using red ink instead, because blood makes terrible ink. When he joins the company, Tyrion mixes some of his blood into the ink he signs with.
Stannis Baratheon uses his own blood to sign an accord with the iron bank of Braavos, because his ink has frozen.
In Brazilian book A Droga da ObediÍncia, one of the protagonists is captured inside the school. Just before being dragged out of the place, he asks his kidnapper to go to the bathroom, and uses the contents of a toilet to write a Morse code message to his friends.
In A Brother's Price Jerin was burning a letter when people came through secret passages and abducted him. He struggled for long enough to be able to write one three-letter word in soot before they hauled him away, the name of the one he thought responsible. Many characters in that world are illiterate, and they didn't have time to clean up as his bodyguards were trying to break through his locked door.
In The Laundry Series the Official Secrets Act and other non-disclosure agreements have to be signed in blood — a sterile lancet or disposable needle and Sharps container are thoughtfully provided.
Live Action TV
Happens from time to time on NCIS, generally of the giving-a-clue-as-to-the-murderer variety. Except for the time it was a call for help. Unintentionally aimed directly at a NCIS investigator... Which also ID'd the person making that particular call for help.
Mitchell: Paint, and it's still wet. Tch. Blood. You ever try writing anything in blood? It's totally impractical.
From Becker: "Look, writing your name in the snow with your pee is good drunken fun when your name is something like Joe Smith. But, when your name is Stanislav Kasacinski and it's ten below out, you're just frostbite waiting to happen."
Red Dwarf has a dying Red Shirt scrawl a warning using blood and intestines. The Cat wonders why he went to the trouble of using his kidney as a full stop. Rimmer notes that it probably just "plopped out" on its own. In Better Than Life (the book), the crew get stuck in a virtual reality game and the only way to communicate with them is by carving messages into their arms.
There's an episode of Friends where Phoebe writes a phone message on the back of Chandler's neck.
"Get the woman a pad! A PAD!"
Jeremy Clarkson declared that the Dodge Viper was a car "so sophisticated, it could write its own name." He then proceeded to write the word 'Viper' on the test track, using skidmarks.
Angel used this trope a lot for ghosts. In season one, a malevolent spirit wrote messages on the walls of Cordelia's apartment. In season five it was the messages in the condensation of Fred's shower door.
Legal documents at Wolfram & Hart are shown being signed in blood.
Jonathan Creek used the contract-signed-in-blood version in the season three episode The Curious Tale of Mr Spearfish.
A classic Sesame Street sketch has Ernie writing a shopping list with chocolate pudding, because he couldn't find a pen, a pencil, a crayon or a typewriter.
Bert: He's improving. Last time he used spaghetti sauce.
Seen in the Torchwood episode, "Captain Jack Harkness." Tosh and Jack are stuck in the past, and need to send a message that will last the next 60 years and be found by the other members of their squad. Tosh doesn't have a pen, so she cuts her hand open on a rusty can and writes in her own blood. Also seen in "They Keep Killing Suzie." The word "Torchwood" is written in blood on a white wall on a crime scene.
Doctor Who is a fan of this trope, as well. River Song, in particular, once used a blowtorch to write "Hello Sweetie" in high Gallifreyan on a starship's black box to be found by the Doctor 12,000 years later. She also carved a message on the diamond cliffs of planet One, making it the oldest written words in the history of the universe, because the Doctor wouldn't answer his phone.
Amy and Rory use a car to make a crop circle reading "DOCTOR" in order to get his attention in "Let's Kill Hitler".
The opening episode of Season 4 of The X-Files had X, Mulder's then-informant writing a message in blood on Mulder's doorstep, having been shot trying to bring information.
In Buffy the Vampire Slayer, after Angelus kills Jenny's uncle the day he had sex with Buffy, he writes a message to Buffy on the wall — "Was It Good For You Too?" in the victim's blood.
Played for laughs in this comedy sketch by Jinnai Tomonori.
In the Charmed episode "The Power of Three Blondes," the Halliwell Sisters are replaced by three evil blonde impostors who are all Brainless Beauties. While impersonating Phoebe, Mitzy Stillman gives Phoebe's newspaper-editor a report written in eyeliner (she was intentionally trying to get Phoebe in trouble with her boss). When explaining why it's written in eyeliner, Mitzy claims that, you guessed it, "I couldn't find a pen."
In an episode of Bones, Bones was being framed for the murder of a friend incarcerated in a mental hospital. It eventually turned out that the victim had left a long and complex clue by writing in his own saliva which only showed up under a blacklight when Hodgins and Cam search his room looking for a clue in a triangle.
The Sarah Connor Chronicles had one of these which was even more absurd than the usual examples of this trope. In the beginning of the second season, a guy makes the jump from the future to the past but is shot when he does so. He needs to get his message to Connors, so what does he do? He breaks into their basement and scrawls a confusing and incoherent message on the wall with his own blood then visits upstairs, whispers something cryptic then dies. He sure failed at that mission.
In the pilot episode (Days Gone By) of The Walking Dead, Rick comes across an abandoned farmhouse where the two former inhabitants had scrawled "God Forgive Us" in blood on the wall before committing suicide.
A sketch on Blue Collar TV had Larry the Cable Guy calling Information for the number to 911 and, not having a pen, bit the tip off a strawberry to write it on the wall.
Starsky & Hutch has two opposed examples. In the episode "Bloodbath", Starsky is kidnapped by a murderous cult, who leave his name scrawled in blood on a mirror for his partner to find. In "The Plague", Hutch is in an isolation room with a fatal disease; before Starsky leaves to go look for a cure, he uses a borrowed lipstick to write his name on the observation window where Hutch can be reassured by it.
Luther. A Satanist killer abducts a woman from her home and leaves the corridor leading from the front door covered in words written in blood such as DO NOT FEAR THE ABYSS, I AM THE ABYSS. Likely a deliberate use of the trope to add to his reputation and creep people out. Also a deliberate taunt to the police, as the blood had been kept frozen from a murder the police had been unable to pin on him years before.
On FlashForward, when Lloyd Simcoe is kidnapped by Flosso, he tries to write a note like this, using his blood as ink and a flyer for paper. It's blown away though, and only found after he's rescued.
In 2point4 Children Ben witnesses a hit-and-run and rushes into the kitchen shouting the car's registration number (license plate) over and over so he won't forget it. Not having a pen, Bill writes the number on a cucumber with a tube of mustard.
CSI NY did a variant where the victim didn't write the message-it was written by the killer in an attempt to implicate someone else.
An episode of the Live-action Largo Winch shows a woman joining a group of villains, and signing her name with her own blood.
The angel-banishing sigil on Supernatural does not count, as it has to be drawn in blood or it doesn't work. However, there was an episode where Sam drew a devil's trap (which does Exactly What It Says on the Tin) in his own blood for lack of any other medium.
An episode of Castle starts out with a murder victim who wrote "LIE" on the ground with her blood as she lay dying. Turns out they were looking at it upside-down: it was actually the number "317".
Premier episode of BBC's Sherlock, where a woman clawed a vital clue into the floorboards.
TheMentalist has the main bad guy, Red John, write a symbol on the wall with his victim's blood, which drove the main character to work with the police to try to catch Red John.
Dungeons & Dragons module I12 "Egg of the Phoenix". A revenant [undead creature] will try to communicate by writing a message using its own decaying flesh.
In the Planescape setting, a cleric of a god of communication once tried to use magic to communicate with The Lady. After The Lady's shadow had passed over her, the cleric arose as a vampire that was unable to communicate in any way except by using people's intestines to form words. To compound the problem, a rumour arose that The Lady had indeed said something to the cleric, leading to someone actually seeking her out to find out what. She was apparently happy to comply, but ran out of guts before she could finish.
Warhammer 40000: The scribes of the Grey Knights chapter use blood to write the true names of daemons, as apparently using ink gives the demon some power over the writing.
Abyssals in Exalted have a Charm that allows them to write in their own blood without suffering any harm from the blood loss.
There's a power that Harrowed have in Deadlands that lets them create spontaneous writing on any surface. The precise effects vary, but making it look scrawled ever-fresh blood is one of them.
At least three times in the Ace Attorney series there has been victims who wrote names, presumably of their killers, in blood. Twice they were incriminating Maya, and both times they were fake.Apollo Justice has a victim writing a number on the floor in his blood, which is then cleaned by his assassin because it was a clue that the victim was an Interpol agent.
In the case concerning the SL-9 Incident two years before, Neil Marshall apparently wrote Ema's name in blood on a very unusual vase before he died... but then it turned out it was actually Daemon Gant, planting false evidence to blackmail Ema's sister Lana.
There's also the case concerning Maggie, whose boyfriend wrote her name in the sand next to him with his right hand before he died. He was left handed, and her name is actually spelled "Maggey".
In the demo case for Gyakuten Kenji, another prosecutor finds that the victim (supposedly) wrote Gumshoe's name in his own blood on the crime scene. As Edgeworth, you have to demonstrate that the detective must be innocent.
In Chrono Trigger, you find a fallen knight who gives you a monster-fighting tip using his blood as the ink. Good thing he was a knight instead of a writer of strategy guides...
Dead Space has "Cut off their limbs!" written in blood around the ship. It's good advice.
Portal has loads of insane messages scrawled in... Uh, it's hard to tell what it is actually.
According to the webcomic tie-in for the sequel, it's just paint.
BioShock has a number of messages written in blood, most notably the message on the board of photos reading: "Would You Kindly"
In System Shock 2, there are walls with bloody messages urging players to REMEMBER CITADEL.
When you reach Delta Labs 4, the epicenter of the demon invasion in Doom 3, the walls are streaked with bloody messages. Whether it was done by demons, demon-possessed victims or just by insane victims, there are no survivors left by the time you arrive. The words repeated over and over again are "suffer", "die" and (appropriately enough for being on the edge of a hellgate) "burn".
In the Mass Effect 2 DLC Arrival, the wall of a prisoner's cell (with the body of the prisoner still in it) has a drawing of the unmistakable silhouette of a Reaper, in human blood.
In Wing Commander Prophecy, it's not shown, but in a discussion between some Non Player Characters in the pilot's lounge, it's said that the Kilrathi aboard the kat fleet that got wasted earlier in the game used their blood to write "Knathrak", roughly equivalent of Ragnarok for them.
At the end of Assassin's Creed, Desmond gets to see some elaborate drawings made by Subject 16 in blood. There are so many of them that one has to wonder how he managed to keep the drawings so neat. Although the 'couldn't find a pen' bit is subverted by the fact that a pen was what he actually used to kill himself. He painted the walls with blood because even after being cleaned away, he knew it would still show up in Eagle Vision.
As Sayaka Maizono from Dangan Ronpa was dying from being stabbed by Leon after she fails to kill him, she uses her own blood to leave a Dying Clue and help clear Naegi's name.
FUG's calling card in Tower of God after the murder of Edin Dan and Gyetang was to write the organization's name in big bloody letters on the wall.
Parodied. When the cast of Ansem Retort played Pictonary, Axel drew his in Riku's blood for kicks.
The introduction of the girls in College Roomies from Hell!!! has Roger slipping a note under their door. Unfortunately, he forgot to bring a pen, so he wrote the note using blood produced by a convenient papercut. Also, the cut stops bleeding before he can finish writing the message, so what the girls find reads more like a death threat from a stalker than an invitation to dinner (Which is what the note was intended to be).
In Homestuck, Vriska mind-controls Tavros into writing text with her blood... for him to read.
And then he wears his fingers down, and the writing becomes in his own blood.
In Juathuur, Rowasu uses blood at the end of chapter 11.
A particularly awesome moment in Kagerou has Dark, bleeding from his eye sockets, writing the names of Red's victims in blood, as well as revealing Red's true name, James Valentine Beethoven.
A nanotechnology reservoir in Last Res0rt is hidden behind a door with some warning scrawled in blood in an alien language.
In Schlock Mercenary, Kevyn Andreyasn uses his blood to write a warning for Captain Tagon, about Kevyn's antimatter grenade epaulet being armed, as the same injuries that gave him blood to write with also prevented him from being able to speak.
Superego has Juliet do this with milk-based paint, and she does it often, driven by her desire to tell others about her psychic dreams and their potential meanings.
Slightly done in Draw With Me, where, oddly enough, they use coal on glass, which makes no sense.
However, it could have been black chalk, and we don't know exactly what the wall is made out of. (It's some durable glass, if it even is some sort of supernatural sentient self-repairing glass.)
Mentioned in Freeman's Mind. While walking around some dead soldiers, Gordan thinks to himself it would be funny to write "Latin is a dead language" on the walls with the blood of one of the soldiers, and make it look like he did it as his final act.
In Batman Beyond: Return of the Joker, a scene involves Bruce Wayne being found after an attack by the joker with "Ha! Ha! Ha!" written on the ground in red. It was changed to purple in the censored version to avoid looking like this trope.
In "Cape Feare", Sideshow Bob writes death threats to Bart in blood. And his diary. And amusing letters to Readers Digest. He faints mid way through one of the latter.
Snake: Use a pen, Sideshow Bob!
Subverted when the family goes over the threat letters and finds one in pen; Homer admits he wrote that one after Bart somehow tattooed "Wide Load" across Homer's ass.
In "The Springfield Files" Homer runs away from an alien screaming "Yahhh!" As we watch from above we see him run through a field spelling out the word "Yahhh!" in cursive (including the exclamation mark, which he dots!) 
The Marquis De Sade, after being imprisoned, wrote his stories on his own body, first using wine and a chicken bone, then his own blood.
The Manson Family had a creepy habit of writing things on walls at murder scenes - in the blood of the victim.
The murder of Frenchwoman Ghislaine Marchal in 1991 involved her writing in her own blood on a wall: "Omar m'a tuer" ("Omar killed me", with a glaring spelling mistake†− despite the victim being a French teacher.) The phrase is still very famous in France (perhaps due to the controversial nature of the whole thing: The case is still sort of unsolved).
Apparently, several people have made last-minute testaments in this way: There is a story about a farmer, trapped under his own tractor writing on the bumper, with mud, who of his neighbours would get which of his animals and about a dying man who wrote "all to wife" on the wall in his own blood. Both of these were accepted as valid.
A man in Northpinellas, Florida tried to invoke this trope after murdering his girlfriend by writing the first name of his victim's ex-boyfriend on the wall in her blood, creating the impression that she wrote her killer's name as she died. His ruse failed when the police took a closer look at the victim's injuries and realized that she would've been physically incapable of pulling it off.
Geocachers who hike several miles to a cache to realize they left their pen in the car have been known to sign the logbook in blood. The preferred way is to make a mark with dirt or mud and explain it in your note online.
Apparently, this was once inverted: The murder victim had a pen and wrote the killer's name on her body. Unfortunately, the conditions the body endured made it unreadable until the FBI's Special Photo unit used an infrared camera to get a better image. The guy was caught.
When Fracisco Pizarro - the Spanish conquistador who led the conquest of Peru - was assassinated, he was stabbed through the throat. Reportedly he drew a cross in his own blood and kissed it before dying.
When the last Mughal Emperor (who was a noted poet) was exiled from Delhi by the British for supporting a major rebellion, he wrote poems on the walls of his room with a burnt stick.