Yeah. I think Bernie dropped the ball in two areas.
1) not doing shyt during trayvon and all the others who died. We keep getting beat over the head about how he marched and walked decades ago but ANY visible action during those times would have given him name recognition and probably at minimum close to the amount of black support Hillary has.
2) calling Obama weak. Hillary never gonna let anyone forget that and it's self explanatory for why he is losing black votes too
I think the first one is bigger.
But him calling Obama weak was a frustration out of the stupid budget crisis with the Republicans in 2011. And Bernie wasn't lying! Obama turned his backs on progressives during his first term.
By all accounts, Bernie and Obama are on good terms and Hilary hasn't used that attack much since the first couple times.