While Hillary Clinton aides don't acknowledge it directly, her momentum from a win in Pennsylvania may be blunted on Tuesday by one of the persistent problems of her candidacy: her struggles with black voters. While polls show her effectively tied with Barack Obama in Indiana, she would gain more among both delegates and in the popular vote if she won the other state voting on May 6, North Carolina. But experts expect that more than a third of the voters in the Tar Heel state will be black, and according to National Election Pool exit polling, Obama has won by...