Biden won the black vote by threatening to take away their blackness.
Remember that blacks are extremely superstitious. The vast majority of them believe in nonsense like ghosts, voodoo, and fortune-telling. So when Biden said that those who didn't vote for him "ain't black" (note that he was careful to say "ain't" instead of the grammatically correct "aren't" — he wanted to make sure they understood him), blacks took his comment literally. They feared the loss of not only their pigmentation, not only their cultural identity, but the loss of every advantage society bestows upon them simply for having dark skin.
Vote for me, or your race card goes away.
Vote for me, or your welfare goes away.
Vote for me, or affirmative action goes away.
Vote for me, or your freedom to hunt white people for sport with no risk of being charged with a "hate crime"... goes away.
In other words, Biden spooked the spooks, and that's why he won. But you'll never hear a peep from the media about his terroristic tactics. It goes against the narrative that the Democrats love minorities and are unstained by racism — not like those evil, evil Republicans.
The above cartoon shows what blacks actually believed would happen to them if they failed to vote for Biden.