Light corn syrup
red food dye (or cherry kool aid)
1. Combine light corn syrup with red food dye until you get a shade that closely resembles real blood
You can also use a red drink mix (like cherry Kool Aid® powder) instead of red food dye, although be warned that this changes the flavour.
2. Carefully add a small amount of cocoa powder to the syrup mixture to darken the shade and cause the fake blood to turn opaque like real blood.