It's not that they dislike Bernie, they strongly prefer Hillary. Prior to President Obama, her husband was, by far, the most popular president ever among black voters. Some of the support may be supporting Bill by proxy, some may be the feeling that she legitimately has the interests of black voters better in mind.
Bernie has a huge uphill battle to erode into black voter support that leans STRONGLY toward Hillary. He certainly has policies that help black voters from an economic standpoint, and probably has support for policies that help black voters from a social justice standpoint, but he hasn't effectively communicated those yet.
At an even more simplistic level - he's another old white guy in a suit. That image hardly screams "change" to low information voters - and they're plentiful on both sides.