Rustproof Blood

Look on the bright side: You'll find another in a few scenes.

Blood is always red. Except when it isn't. Human blood, anyway. This is true even though blood fairly quickly curdles and scabs to a brownish color when exposed to air, to the point that chocolate syrup is often recommended to fake scabs. Even literary description of the stains, if they are not shown, must specifically say that the stains are of blood in order to be understood. This is probably done as a bit of visual shorthand, to let the audience (and other characters) know that the stains in question are actually blood, as opposed to other things that stain brown, such as mud, chocolate or feces.

A similar error is to have fresh blood dilute when washed from red to a pinkish colour. Due to the composition of blood (mostly red cells floating in plasma) diluting it with water actually turns it yellow.

Bloody Murder is, oddly enough, entirely unrelated.

Examples of Rustproof Blood include:

Anime

Angel Beats!, pictured. Otonashi's shirt is still sopping wet with bright red blood after he recovers from being stabbed in the chest in the clinic. Even with the Healing Factor, some of it ought to have dried. Or, you know, reabsorb into his body, since that happens at a different point soon after.
Appleseed has a rusty patch on the floor in an abandoned science lab. It isn't until a 3D security video is replayed that you realize it's dried blood.

Film

Despite only having had a single bucket of blood dumped on her, the titular character of Carrie remained red and dripping as she walked all the way home. Then, due to the type of fake blood used, when she washed it off in the bathtub it inexplicably turned pink.
- And despite sitting around for hours, the blood hadn't coagulated or turned brown in the bucket before being dumped on Carrie's head. (Maybe a preservative of some sort was added?)
- In the novel, it had been transported in buckets of ice, and had frozen in transit, to thaw between placement and deployment.
In The Brothers Bloom, the visual difference between real blood and stage blood clues a character that they're being conned.
During the opening sequence of the movie version of Stephen Sondheim's Sweeney Todd: The Demon Barber of Fleet Street a large amount of blood goes through several gears, into a sewer, and eventually washes out to sea—it froths and coagulates a little, but stays a dark pink/pale red the entire time.
In the Agatha Christie book and movie Death on the Nile, Poirot finds the murder weapon (which has been chucked into the titular river) wrapped inside a cloth. The mysterious pink stain on the cloth leads Poirot to suspect that one of the passengers faked a bloody injury (using red nail polish) in order to create an alibi for himself.
The blood on Coulson's vintage Captain America cards in The Avengers...which turns out to be the first clue that Fury didn't find them where he said he did. They were in Coulson's locker when Loki shot him. Whether that means Fury just added the fake blood for extra punch or something else has yet to be seen.

Folklore

The redcap is a particularly vile goblin from British folklore that waylays you on the road at night, kills you, and soaks its hat in your blood; really it should be called the browncap. (Of course, it does go out of its way to get fresh "dye" every night...)
- Ursula Vernon's Little Creature and the Redcap deals with a redcap's frustration with the nature of realistic blood.
- Some versions of the myth say the redcap dies if the blood on its hat dries out or washes off. Which means it must have to re-dye its hat every hour or so.

Graphic Novels

Averted in Johnny the Homicidal Maniac:

"Blood changes color when it dries! Don't you see? I have to keep the wall wet!"

Literature

In the Merry Gentry series, certain Redcaps (see Folklore above), usually the most powerful ones, have the magical ability to keep the blood on their hats from drying out and rusting.
Averted in Nick O'Donohoe's The Magic and the Healing. The Big Bad's fascination with washing her hands with blood moves her to kill a lot of things, because the blood never stays fresh and red for very long.
Averted in the Heralds of Valdemar series. The robe of the Emperor of the East is described as the red of freshly spilled blood. The uniforms of his soldiers is described as the reddish brown of dried blood. The soldiers joke that this is so that you can't tell whether or not they've gotten blood on them, thus reducing laundry bills (It may even be the truth).

Live Action TV

CSI
Law and Order
The pool of blood in which Dexter was found seems to have been made of this trope. Crime scenes he investigates as an adult also seem prone to this.
- Dunno about the crime scenes, but the blood in the pool had been refrigerated and mixed with anticoagulants.
  - Not that; the blood in the storage tank where he was found. There was a lot of it, but you'd still think it would've started to scab after what- two days?
The suspicious redness of a bloodstain was a vital clue in an episode of Jonathan Creek.
The Wire falls into this trope, despite being otherwise brutally realistic. Not a drop of blood turns brown, despite the fact that the show frequently shows the murder, the investigation, and how much time has passed in between when the blood should have turned brown.
Lampshaded in an episode of Millennium. Two people working on a slasher movie were fooling around with bags of fake blood shortly before they were murdered. This caused the crime to appear much more violent than it actually was. As Frank Black arrives at the scene he soon tastes some of the blood (to the puzzlement and revulsion of others present) and comments that it is fake blood, which he already figured out since real blood turns brown as it dries.
Lampshaded in the "Tuesday the 17th" episode of Psych. While searching for a camp counselor who's been missing for days, Shawn, Gus, and the other counselors find her bloodstained pajamas in the laundry room. Later, Shawn comments that real blood would have dried and started flaking off by that point, which clues him in to the fact that it's all a hoax; the counselors are beta-testing a murder mystery/horror weekend they intend to hold at the camp.

Video Games

True of every bloodstain ever appearing in the Phoenix Wright: Ace Attorney series. Or if not all, then at least most. This occasionally causes problems for one character, who can't distinguish between red and white.
In 99% of violent video games, old bloodstains might be, for example, reddish-brown at the darkest; and new blood will be red and stay red no matter how long the corpse lingers.
- An exception is Uncharted: Drake's Fortune; Drake comes across a beached U-Boat, whose crew were really torn up by something. Over the decades, all the blood on the surfaces and corpses has faded to a rusty brown that's not even recognizable as blood unless you know what to look for. The game also averts the similarly universal Soft Water.
- And Silent Hill, where sometimes you must question if the walls are covered in rust or dried blood... as well as the fact that some monsters bleed blood that looks almost brown.
In BioShock (series), not only are all the blood stains bright red - many of which must have been there at least a year - they're still wet and shiny!
In Dragon Age, blood never seems to dry. Maybe warriors clean their armour off frequently, but one would assume mages in cloth robes/light armour or rogues in light/medium leather armour would have some trouble getting all the stains off.
- A mage washed them.
- Also, you just keep getting hosed off with more blood in every single fight.
Example from The Legend of Zelda: Ocarina of Time. The bloodstains on the Shadow Temple's torture equipment looks bright and fresh—although this could just be an apparition, what with it being inside a magic temple and all...
- Or, to up the ante on the Nightmare Fuel quotient of that place, it really could have been fresh...
Masterfully averted in Fallout 3. The town of Minefield is full of decayed corpses. However, as the player searchs for mines they might spot a trail of bright red blood leading to a house, with a bloody red palm print on the door. The fact that this blood is fresh is a clue that the player is about to be attacked by a sniper.

Web Original

In Death By Cliche, a villain's carpet is described as "brown, not because it was a tasteful colour, but because eventually blood dried."
Inverted in the Fewdio short horror piece, "Cleansed". The thicker pools of blood turn dark and sticky while the lighter stains darkened to brown.

Real Life

Reality Is Unrealistic: The Heeresgeschichtliches Museum (Army History Museum) in Vienna houses in a display case the uniform that Archduke Franz Ferdinand was wearing when he was assassinated in Sarajevo in 1914. Complete with bloodstains, which have bleached white over the years. This is because of the light, as the colour red fades over the years due exposure to sunlight.
Bloodstains are often so similar to the rust stains that it's impossible to distinguish them unless a proper chemical analysis is done. There were a lot of cases when they were confused for each other, including one during an original Mary Celeste trial. The prosecutor insisted that the brownish stains on a saber, found on a ship, were blood, until the judge ordered them to be analyzed, where they were proved to be just the rust.