There's been lots of speculation about the reasons for Bernie's apparent lack of support among black voters -- but has there been any analysis of the extent to which black voters support the perceived frontrunner in Democratic primaries? Even Obama didn't gain traction among blacks until the narrative shifted and he was the candidate with momentum.
While there are of course other reasons in play (especially considering the number of reasons particular individuals have for supporting various candidates; it's not as if all black people share a hive mind), I'd speculate that Hillary's black support largely comes down to two things. The obvious reason is that black people simply know Hillary better (and have forgotten about and/or forgiven her for some of the awful things she said during the 2008 campaign and earlier). But the other? I think black people simply don't want to "waste" their time and emotional energy on a candidate who's probably not going to win -- and I think black people (like most people) like having bragging rights for having supported a winner.
To draw a sports comparison, politics is like a sport that many black people aren't especially invested in. When people don't especially care about the outcome of a game -- in the absence of other reasons, anyway -- they're probably more likely to support the favored team.
Pure speculation, of course. (I think these reasons in part explain Trump's support, too -- and are potentially what will allow Trump to attract black voters.)