There is a great book that deals with subjects like this. It is called Zombie CSU by Jonathan Maberry. Blood in a dead body doesn't clot, it actually turns into a lighter fuid as most is absorbed by the tissue, which would still make for a nice splatter. You can look at the zombies in Return of the living dead to see how they portray interred(burried) corpses being shot. Most of the time it is a cloud of dust and some fluid which seems about right to me. However, if it is a viral thing the rules would be different. The beauty of zombies is that whatever you want to imagine pretty much goes, so long as it's a good story and within a reasonible realm of reality people will prolley some what like it.
This would depend on the reanimation period. If it's slow, the blood would congeal in various places in the body. Of course, the virus could turn blood into the viscous black substance that Brooks mentions in one of his books.