His overall approval has basically bounced back and forth between 47 and 52 the entire election year . . . I think you are just overreacting some. Black voters are 12-13% of the population. a 10% move by them only changes the needle by about 1.5% of the total.
If 20% of Blacks vote for Trump, it will give him a comfortable win.
A sharp increase in black support for Trump would result in a Trump victory.
If I recall correctly, Trump got about 8% of the black vote in 2016. Romney and McCain got in the neighborhood of 5%, I think? Republicans in recent decades have gotten less than 10% of the black vote.
A sharp increase in black support could mean Trump wins states such as Michigan and Pennsylvania easily, rather than being close.