I think you're misreading my statement, it's a 10% swing in the direction of Trump, not a 10% overall preference for Trump. From 82% favoring Hillary to 71% favoring Biden for black voters. That's a 10% change towards Trump's direction. If 16 million black voters participated in 2016 then that's around 13.1 million votes for Hillary and 2.9 million for Trump. If polling is correct this year and we see around similar turnout (not even an increase), then it'll be around 11.3 million votes for Biden and 4.7 million for Trump. So a 1.8 million vote swing in the black vote.
That's just a really rough calculation and doesn't account for the Hispanic vote either in that article.