we can update those numbers with something more precise. the margin is roughly the same because the concept is just shifted, but there is a conceptual error in the previous post as a consequence of the confusing language; it's 40% of all voters that voted clinton and were black, not 40% of her voters that were black. i did it right the first time if you go back and look...

this is the correct and more up to date math. it's the same conclusion, which is what i pulled out intuitively and then sat down to actually calculate, but the numbers are bigger because they represent correct proportions.

clinton in 2016:
(61% of all voters were black)*(86% voted for clinton) = 52.46% of all voters were black and voted for clinton.
373,063 total votes were cast. so, she got 195709 black votes. roughly.

biden in 2020:
(56% of all voters were black)*(61% voted for biden) = 34.16% of all voters were black and voted for biden.
527,728 total votes were cast. so, he got 180272 black votes. roughly.

that's a decrease of 15000 votes. roughly. 

....amidst a huge increase in turnout, and what i believe is substantive population growth.

where did all of the new votes go? well, sanders went down, too.

they appear to have gone to the lower candidates. so, i may have been on to something, it just didn't catch critical mass.