A substance that appears to be blood, but isn't - in color artworks, commonly ketchup, red bean paste, jam or paint; though monochrome ones might also use other substances, like chocolate.
Not every instance of a red fluid is necessarily faux blood - look for crazed expressions, knives, signs of "injuries", or people lying lifelessly on the floor (with the source of "blood" visible).
See also
Ambiguous red liquid - when it cannot be determined if the liquid is blood or not
