Black turnout dropped from 30% in 2004 in South Carolina to just 25% in 2008. It's amazing that happened with Barack Obama on the ticket!
Well actually it didn't. But it's a good reminder of why exit polls should not be treated as the gospel truth.
South Carolina happens to be a state that makes the actual turnout data very easy to access. In reality the nonwhite vote increased from 27% in 2004 to 31% in 2008. While the exit polls erroneously tell the story of a five point drop in the black share of the electorate the reality is that it increased by four points, as you would have expected with Obama at the top of the ticket.
Exit polls aren't completely worthless, but pollsters definitely shouldn't weight their samples to them and their numbers should be looked at with the same skepticism that poll observers would show toward any other survey.