The indians don't count. There are more black votes than indian votes come election time.
The minute increase of black votes for Republicans in 2016 could be attributed statistically to lower turn-out for Democrat-supporting blacks.
In other words, Democrats aren't leaking black votes. If Democrats are doing things that "favor" blacks, it's not to get votes they already have. The George Floyd murder motivated black turnout in 2020 (undermining Trump's prediction he would get 95% of black votes).
2016 was a year in which voters in general were fed-up with traditional politics, exemplified through "gridlock" and partisan-bickering on Capitol Hill. It favored the rogue "outsider" candidate. That feeling still persists (Trump got 74 MM votes) but after Jan. 6, the greater danger is indulging authoritarianism and Far-Right Fascism.