Black voters in the South maintain an effective "veto" over the Democratic nomination process because of DNC delegate allocation rules that make Southern states relatively delegate-heavy. No Democrat has won the nomination without securing the bulk of Southern Black's support - not Hillary, not Kerry, not Gore, not Bill, etc. In that sense, Hillary was very smart to tailor her message and campaign to Southern Blacks.
To answer the broader question, Black Democrats (especially in the South) see themselves as much more the beneficiary of activist government than White Democrats. For this reason, there is a propensity for Southern Blacks to gravitate towards more pragmatic, electable candidates because they have too much to lose by supporting an ideologue like Bernie Sanders. White liberals don't have the same skin in the game, so to speak.
Very good point. Minorities are much more likely to be poorer than whites. Do you think the average Black Democrat in Alabama is going to care much about transgender rights or global warming? No, they are much more likely to care about pragmatic issues that actually effect them like a higher minimum wage, healthcare and voting rights/civil rights. White wealthier Liberals have the luxury to ignore those more basic bread and butter issues and focus on other more esoteric things.
