If he doesn't explicitly mention African Americans in every speech, he's ignoring Black issues. If he explicitly mentions African Americans in every speech, he's "Bernie-splaining". It's clear Bernie could do nothing to please the people who write articles like this, so he shouldn't try.
I hate to empty-quote Mortimer but he's right. While some articles written about Black voters' love of Hillary are quite condescending, this is true of progressives in general, who have the tendency to make claims about "interests". "If only Group X could see where their interest would lie, they would vote for Candidate Y." Articles in this vein have been written about the white working class time and time again. This has nothing to do with racism, it's related to a certain tone-deafness that exists in certain quarters of the left.
That said, it is fairly accurate that Black voters know little about Bernie Sanders. There's nothing condescending about stating this; they tend to make less money and to have lower levels of education. They're also concentrated in states where Sanders has run little advertising or radio ads. They might know his name and understand his positions on a few issues but I don't think that, for the most part, they have been paying much attention to the primary. Does this mean that they will ultimately end up backing him in large numbers? I doubt it but I don't think that Clinton-luvrs can afford to be so complacent.