This Wall Street Journal article reports this, with some detail:
"Barack Obama’s rising poll numbers among white voters in Iowa and New Hampshire are having an unexpected ripple effect: Some black voters are switching their allegiance from Hillary Clinton and lining up behind him too. That could mean a further tightening of the Democratic presidential race, especially in southern states where blacks make up as many as half of Democratic primary voters. The evidence of movement is most clear in South Carolina, site of the first primary where black votes figure to make a significant impact. There, four polls now show Illinois Sen. Obama with a lead among African-American voters for the Jan. 26 vote. As a result, the race in South Carolina has tightened, with some polls calling it a dead heat."