The fact of the matter is that if Bernie Sanders won the Democratic nomination for the president, which is still unlikely but not impossible, blacks would overwhelmingly vote for him in the presidential election. The simple reason for this is, historically, blacks have voted in accordance with their economic interests.
The same is not true for whites. For a number of reasons, many which are rooted in racism, a very large number of whites have voted for people whose political program is to make life worse for them, and whose economic interests are aligned not with them but with their bosses. And a whole party is sustained by these voters.
This is why Sanders is really only preaching to white people. Despite the obvious facts of stagnant wages, job insecurity, weakened social services, massive white poverty, and the increasing concentration of wealth, white voters have to be won over with speeches, rallies, and headlines in the mainstream news. Blacks do not need to be deprogrammed or enlightened by Sanders. Their votes always go to the best deal that the broken system has to offer. That's not to say it always works out, but they tend to have a better idea of who the enemy is in the class struggle.