My assumptions about a black Republican candidate were pretty well dashed by Ken Blackwell's thrashing in 06 in OH. Not to say this is the case in every state, but OH still has a lot of blue-collar whites who aren't yet ready to make the leap to a black leader---Obama or otherwise. While Obama would lose their votes in a place like OH or W VA, or KY, I don't see any black Veep "gaining" them for McCain.
The only reason now I would even consider voting for McCain is if he picked a young conservative as his veep.