We've seen Hillary's problem with whites. In the 26 state primaries and caucuses where exit polling was conducted in both '08 and '16, this time around she received fewer white votes in 23 of them than she did when she lost the nomination to Obama in '08.
But the Democrats have a black problem, too. Across those same 26 states, black turnout in '16 was down 11.1%--over 600,000 votes--from '08.
It's not as dire a problem as the party's white flight is. White turnout was down 26.5% in '16 compared to '08 in those 26 states. It could end up being the difference in states like Michigan, Pennsylvania, Ohio, and Florida, though.
The Democrats' runaway whites and their unenthusiastic blacks are two major problems Hillary will face in November.
Parenthetically, over the course of Obama's presidency the Democrats have been making progress towards successfully (re?)branding themselves as The Black Party. There was one black voter for every three white voters in the '08 Democrat primaries and caucuses. In '16 that changed to one black voter for every 2.5 white voters.