I think the divide is highly overstated and is more a reflection of the specific relationship that the Clintons have with black voters from the 90s and the Sanders campaign approach to try to focus on states they felt they could win, which based on the polling at the start of the primaries were states with high white liberal populations, rather than investing in trying to limit losses in southern super Tuesday states. I don't think this is a racist strategy as the headline after a primary is Candidate A won, not Candidate B lost by less than expected.

I don't think the difference would be nearly as stark if either candidate was different.