Supposedly Biden does better with older black voters than he does with young ones, where as with Bernie the trend is reversed.
https://www.google.com/amp/s/amp.cnn...ics/index.html
That being said the same general trend applies to all races. Biden appeals more to the 40+ crowd and Sanders appeals more to the youth. And across the board Biden currently is the clear front runner in general, and even more so with potential black voters.
Older crowds tend to vote more in actual elections as well - which these polls don't reflect.