I don't know the numbers on this so I genuinely don't know, but are older black voters a reliable voting bloc for Dems in the general because they'll just vote Dem no matter what? Or are they reliable voters in the general because the nominee is typically someone who had already won their support in the primary already? Obama, Gore, and both Clintons won with older black voters in their primaries. Is it a problem for Bernie in November if he wins the nomination without winning over 50+ black voters first? I don't know.
John Kerry actually didn't win black voters in 2004 IIRC, though it was extremely close. I guess we are going to see this fall, because 4 out of the last 5 Democratic nominees won the black vote outright in the primary, but I expect that older black voters are going to show out and vote blue no matter who the nominee is.