(At the root of it...)

I would think that the taboo has its roots in health reasons. (people drinking the blood of animals, that is) All carnivores and omnivores consume at least a little blood of the other creatures they eat. Uncooked meat is potentially unhealthy though. (That's why gravy isn't taboo.) The taboo with people drinking the blood of other people probably stems from a fear of being attacked by another person. It also may be linked to the cannibalism taboo, and linked to the nonhuman-animals-are-lower mentality. You don't eat your own kind, only that which is lesser than you.


Just a guess.