Fake blood needs to be sticky and goopy.
Start with 1 cup of clear corn syrup. Add a few drops of red food coloring and one drop of green to create a nice dark-red blood that's safe and nontoxic. To imitate foamy blood (as it might appear in a sucking chest wound), replace half the corn syrup with clear dishwashing liquid.
[ via P.D., Wired magazine, Oct. 2007 ]