What real evidence is there to support that though? Bernie's message is that the middle class has declined over the past 40 years. That over time money and special interests have gotten more and more involved in lobbying to funnel financial income to the top 1%. That repealing glass steagall and bailing out the big banks was the worst thing to happen ever.
What Bernie is really communicating is that the country was much better, at least economic wise in the 1940s,50s and 60s. When we had a 90% tax bracket, when the middle class was at all time highs and the vast majority of wealth wasn't distributed towards the top 1%, when citizens united or Wallstreet didn't have the same effect on american politicians that they have today, when infrastructure was booming and we were constantly building state of the art airports, highways sky scrapers, etc. What good was any of that for minorities? They didn't see any of it and weren't included in the "good times" because of systemic institutional racism.
Fixing the economy does nothing when institutional racism is present. Raising the minimum wage is fine, but I'm pretty sure minorities would like a lot more than just a minimum wage, or just to get by, or what have you. They want to be included in the system full stop. They, just like everyone else, want to achieve their dreams, become somebody, doctors, lawyers, actors, etc. A system that allows everyone to excel. I don't know how Bernie's wallstreet reform plays into stopping employers from throwing out someone's job application because their name sounds too ethnic. Or how Hillary taking 600k to do a speech negatively effects them.
Had he really driven home those sorts of points, talked about real systematic reform, or reparations even, I can't say for certain, but I think you might see the same sort of enthusiasm towards him amongst black liberals as white liberals.
Its not to say that his record is bad, or that what Hillary has said or done is necessarily better. He just hasn't done or said anything that excites minorities or given them a reason to jump ship in the same way