A character who has an odd fascination with blood, such as talking about how they wouldn't mind drinking it up, enjoying the sight of blood being spilled, etc.. Often, they may be seen licking blood off of their weapons. Very often a sign of an Ax Crazy character, but not always (as seen as in the picture). You still would be(probably)safer if you stay out of their sight, though.
In real life, licking a cut is an instinctive reaction - the saliva can help to clean the wound. Anyone who've ever had a pet will probably have noticed the same behavior in them if they're ever wounded... and unless you deliberately suppress the instinctive reaction, licking or sucking on a minor cut is a perfectly natural response. So actually, most of us probably knows what blood tastes like, without there necessarily being any fetishistic subculture involved. Please note, this can get you very sick.
Despite this trope being titled like a real word, meaning a desire for violence and bloodshed, this trope is not technically related to that definition - the trope Blood Knight is about people who definitely have bloodlust in the sense of the actual word. For when this is a Fetish for a character, see Hemo Erotic.
Examples:
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Anime and Manga
Grell Sutcliffe, Madame Red, and Sebastian from Black Butler.
Ladd Russo of Baccano wears a white suit on board the Flying Pussyfoot so that the blood will show when he starts. When one of his associated is brutally massacred, Ladd dances in glee.
Andrea of Gankutsuou has a scene in the anime where he deliberately squeezes a glass in his hand, breaking it, and causing his hand to bleed. He then licks the blood off his arm.
Barry the Chopper from Fullmetal Alchemist has such a ridiculous obsession with blood that it's humorous.
Takuma Fudou of Get Backers expresses a desire to "slurp up Ban's blood."
Karate Shoukoushi Kohinata Minoru: During their fight Kevin squeezes Minoru's throat hard enough to make him cough up blood, catches some on his tongue, and drinks it. In his words "Delicious flavor...LET ME DRINK MORE!"
Gorobei from Samurai7 has, on at least two occasions, apparently derived pleasure (if not flat-out orgasmed) from suffering cuts.
There is a certain scene in Kara no Kyoukai where Shiki definitely invokes this trope ("It was the first lipstick Shiki ever wore"... you imagine the context), though it was most likely a minute's whim rather than full-out blood fetish.
How can anyone forget Belphegor from Katekyo Hitman Reborn!? The Bloody Prince? Prince the Ripper? The sight of his own blood makes him even more desirous to rip his opponents apart and see their blood. More likely than not, Bel also likes the taste of it. I think it was stated once.
In the first anime, a character named Boobanshee/Bubbancy (actually a corruption of a Baobhan sith, pronounced roughly the same way in Japanese) tricks the entire Hellsing organization, save Seras and Alucard, into thinking that she is Integra's sister, Laura (a reference to Carmilla). Later, she attempts to drink Integra's blood, and even bites and licks her chest, after which Integra realizes that she has no sister; shortly after Bubbancy is dispatched and Integra refers to her as Countess Karnstein (another Carmilla reference). She also licks Integra's blood from her hand while saying that Integra's blood is "delicious".
Arcueid from Tsukihime doesn't necessarily craves blood, but she gets really turned on with something as small as a bleeding gash in the arm.
Bleach: In the anime-only Invasion Arc, one of the shinigami clones is revealed to have this. Yumichika can't attack his clone because he thinks his clone is far too beautiful. However, his clone has no qualms about tearing Yumichika to shreds precisely because he thinks he'd look even more beautiful covered in blood.
Comic Books
In exchange for its massive power, the Witchblade demands blood. While Sarah from the American comic tries to overcome the feeling as much as possible, Amaha from the anime gives in to its insatiable hunger for violence.
Vampire Joker is shown raiding a blood bank, and when he breaks the glasses holding the blood, causing a sort of blood rain, he's immensely pleased (and even laughs in glee!).
The Elseworld Batman & Dracula: Red Rain had a Dracula that wound up oddly Joker-like (although it apparently wasn't meant to be a Joker Expy) who got a taste of the Dark Knight's blood and really, really liked it.
Film and Television
Kari Byron, of all people, expressed an attraction to blood in one episode. But she doesn't taste any, it's just pretty.
The original Dracula movie had a scene where Renfield accidentally cuts his finger and Dracula stares hungrily at the blood seeping from the wound.
Literature
In The Guardians, when vampires drink blood from someone attracted to them, they are overwhelmed with lust and sex is inevitable. However, the choice belongs solely to the blood donor; they may resist or succumb, but the vampires cannot resist.
Some of the more unsettling villains in the Anita Blake series express a profound interest in the red stuff. Only partially justified by being vampiric, as this does not seem to affect the Friendly Neighborhood Vampires to an extent that the author would feel a need to impress it upon the reader.
In Bram Stoker's original Dracula novel, Count Dracula goes from being a charming gentleman to a raging fiend with the flip of a switch — and the switch is Jonathan Harker cutting himself shaving.
Table Top Games
The art for the Magic: The Gathering card Blood Lust (as seen above) is an accidental occurrence of this. The card itself was based around the "Desire for violence and bloodshed" bloodlust, but a miscommunication with the artist resulted in its picture being more in line with this trope.
Warhammer 40000 has the Blood Angels chapter of Space Marines, who have numerous unspecified rituals surrounding blood. Additionally, a genetic defect known as the Red Thirst has a random chance of triggering a usually temporary overwhelming need to kill and spill blood, sometimes leading to a Leeroy Jenkins style destruction of battle plans.
Vega, the masked Spanish ninja in Street Fighter. He talks about how his enemies should "soak in their blood" and that he can hardly wait to taste it. (Why yes, he's evil.) There's also the page quote...
In the original Guilty Gear, Zato's win quote against Millia went along the lines of "Finally! Only your blood can satisfy me!"
Metal Gear Solid 2's Vamp, sort of. He has a blood fetish, due to sucking his dead relatives' blood to survive in the aftermath of a church bombing.
As an example of a good (sort of) person doing this, Rider in Fate/stay night seems to find blood a bit of a turn on and also drinks it. When Shinji makes her, it's generally fatal and to get some strength from it, plus not her idea. However, she also seems to simply like the taste and neatly avoids answering Tohsaka about whether or not she's still doing so by saying if she was, Sakura wouldn't find out anyway. Bloodsucking is a bad thing in the Nasuverse but Rider gets away with it anyway.
In the Suikoden series, several of the True Runes are stated to cause an insatiable thirst for blood in the bearers, most notably the Beast Rune, which ends up causing an enormous amount of bloodshed through its bearer, LucaBlight.
Blood Mage: My blood cries out for the vengeance of my people's blood, which can only be repaid with twice as much blood! Or maybe three times as much blood! Like, if you went to hell and it was full of blood, and that blood was on fire, and it was raining blood, then maybe THAT would be enough blood. Maybe but probably not...
The initial protagonist of Theresia: Dear Emile is deeply comforted by the smell of blood. It reminds her of her mother.
