The only thing that could really push black voters away from Biden specifically on the issue of their race is if Biden slips up and tries to defend or support the tough-on-crime bill that really targeted african americans signed back in the 1980's or 1990's, and even still only if this slip up happens like 2-3 days before the election.
The fact that Biden has openly admitted it was a bad policy to support, that the policy is from over 20 years ago and effectively irrelevant, and that Obama will surely campaign on behalf of Biden to convince black voters to go with him rather than vote for Trump, all combined will almost surely bring back any wavering black support that could break for Trump following his more recent gaffe, and thats not even taking into account that Trump could likely do or say shit that aggravates black Americans just as easily to going back to Biden.
On another note: In 2016 Trump got all of 8% of the African American vote, how much of that was from people who just didnt like Hillary compared to those that actually liked Trump?...... The 2016 election was effectively a referendum on whether or not 'the system' was working out for Americans. 'The System' overall has worked AGAINST African Americans in almost every measurable way, so those wary of 'staying the course' rather then trying a new path could have found some appeal with Trump.,... This time around though, now that we have had a taste of Trump's way of doing things, Trump has to rely entirely on his record more than what could be speculated to happen back in 2016. Any advantage he might have had up to this point went out the window with the Coronavirus causing the economy to go to shit, the virus impacting minorities higher than average, and Trump's weekly emotional breakdowns regularly making him look bad. I wouldnt be surprised at all if in 2020 the percentage of the African American vote Trump gets drops from 9% to below 5%