The black inner cities of Atlanta, Detroit, and Philadelphia could see turnout decrease significantly. Polling indicates black voters are not excited for Biden. He will consolidate them eventually but they aren't guaranteed to turn out in high numbers.
What do you think of the reverse argument that these communities already had decreased turnout in 2020 so there's less room for Democrats to fall? Some seem to argue that lack of in-person campaigning by Dems in 2020 disproportionately hurt them in black communities and so things should revert a bit in 2024.
I personally agree though that low black turnout is more of a problem for Biden than large quantities of black voters flipping to Trump.