At Dragon*Con one time, I was sitting in on a q&a period with
GWAR. Dave Brockie said that the blood they use is red food coloring with a clear ice cream preservative for a base. I wish I could remember what it's called, but it's edible, it doesn't stain SO badly because it thins out, and it doesn't ruin the monitors and amplifiers. :P Try Googling for some info on it, perhaps.