It's not like black voters suddenly are going to vote for Trump more, it's just that more black Biden voters relative to black Trump voters will not turn out, reducing the margin. By how much, we don't know, that we will only know by november. Polls centering around specific demographics are unreliable and useless, esp since a lot of those demographic groups aren't homogenic.
2020 Biden Black voters can definitely vote for Trump more. A lot of the gradual weakening of Dem numbers with Black voters is turnover from 98% Dem seniors to 85% Dem youths, and some of it is differential turnout, but there is also persuasion going on. If Trump gains more than a few percent among Black voters, it will be from persuasion.
I can't claim to understand the mechanics of persuasion for every Clinton-Trump or Biden-Trump voter, but they have their reasons. It is extremely Atlas-brained to act like Trump's gains with non-Whites are due to extreme turnout changes and not persuasion. Even when persuasion is acknowledged it's dismissed as solely from incumbency a lot of the time.
