Chocolate Blood

1/2 cup water
1 tablespoon cocoa powder
3 or 4 tablespoon corn syrup
1/2 to 1 teaspoon red food coloring
2 drops yellow or green food coloring

Preparation:

Mix the cocoa powder thoroughly into the water before adding the other ingredients - it may help to use warm water.
After adding the rest, blend the concoction well, and then wait for it to settle a bit.
Either skim the bubbles & chocolate scum off the top with the edge of a kleenex, or pour the mixture into another container.
The longer it sits, the more the cocoa tends to settle to the bottom, which oddly mimicks the effect of real blood seperating.
If you splatter this mixture onto cloth, it makes neat two-part marks which dry into pretty convincing bloodstains.
If you let it run from a victim's mouth and then let it dry, the blood darkens and cakes to the skin in much the same way real blood does.
I can also say from personal experience that any washcloth used to wipe down the 'bloody' face afterwards looks remarkably realistic, too.