McClatchy Washington Bureau | 05/01/2008 | Will black voters stay home if Obama loses nomination?: "If Obama isn't the nominee, 'there would be a significant number of African-Americans who would stay home. They're not voting for (presumptive Republican nominee) John McCain,' predicted David Bositis, a senior analyst at the Joint Center for Political and Economic Studies, which researches black voting trends."
I can see how this could happen. But on the other hand, is it possible that Clinton could mend fences with black voters in time for the election? I don't know.