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