This is an important question, and it reminds me of something that's been bugging me about AMC's celebrated show The Walking Dead, otherwise a masterpiece of realist dystopianism. But seriously: what is the deal with zombies, when they're violently assaulted with, say, an ax—a not-infrequent occurrence on the show—spurting blood? My own Adamsian investigation brought me to Internet medical clearinghouse Wikipedia, which in its entry on death ("‘Dying' redirects here. For the process of coloring, see Dyeing") claims that one side effect of this common condition is the cessation of cardiac activity. The heart stops pumping, etc. Where is all this undead blood coming from?
Which is to say: if you're dressing as some sort of sexy zombie this weekend, save a couple bucks on ketchup. There's no need to overdo it. You're dead!