On the outside, they may be Human Aliens, completely indistinguishable from us Puny Earthlings. Or they can be shapeshifters. Or Rubber-Forehead Aliens. Whatever...
What is the best way to quickly establish that a character is not human, sometimes to shock other characters around? Dissection? Squick. They may have Bizarre Alien Biology, but that's usually just an informed characteristic which is rarely appealed to, if ever. Unless it's something very obvious, like having two hearts. Solution? Make them bleed. If their blood (assuming it is actually blood and doesn't serve an entirely different purpose in the body) is not red, or nonexistent at all, they might as well have the words "NOT HUMAN" written on their forehead.
The trope, of course, is not limited to Human Aliens (Plant Aliens often have green blood, for no apparent reason), but for other types, it doesn't allow for The Reveal in such an effective way.
With Human Aliens, this is a very mild case of Artistic License – Biology: Since the skin is translucent, a creature with different-colored blood wouldn't have the same skin color as a human...if the skin is white. Darker-skinned aliens, or ones with scales, feathers, hair, or something else hiding their skins avert this problem, albeit probably unintentionally.
Of course, here on Earth, all vertebrates (with the exception of certain lizards and genetic conditions), the largest and most complex members of the animal kingdom, have red blood—the reason? Because all animals breathe oxygen. Red blood cells are red because they contain a protein chemical called hemoglobin, which is bright red in color. This is because hemoglobin contains iron, which is vital for transporting life-sustaining oxygen where it needs to go. There are other pigments like hemocyanin or chlorocurorin that come in such vibrant shades as pink, orange, blue, and green, but these are much less efficient at transporting oxygen than hemoglobin and thus only appear in invertebrates, which have smaller bodies and a better surface-area-to-volume ratio, and thus less need of efficient oxygen transport. Usually hand-waved by having different chemicals in their blood.
In Video Games, aliens and monsters often have different colors of blood than red, frequently appearing as optional Adjustable Censorship, to appease the Moral Guardians. Because it's okay to shoot non-human beings if they don't bleed the same way.
Assuming it's not red like human blood, aliens and monsters most commonly have blood that's either black or some bright color usually associated with something unhealthy, like orange, green, or yellow (blue and violet are possible, but rare).
Not to be confused with Blue Blood or Black Blood, even if it is, in fact, blue or black. (Though some blue-blooded aliens can, in fact, be Blue Bloods.) Also not to be confused with a book of same name by Joan D. Vinge.
See also Machine Blood, which is for bleeding machines/robots.
