Pretty much this. People were backing away from Obama for a time, Biden never really did, and I think that resonated. Plus, no one old enough to remember really holds the crime bill against him since even black community leaders were also supporting it at the time - if you want someone to blame the answer is "No! I'm Spartacus!"
Obama presided over probably the biggest civil rights push (sadly via executive orders and court pushes rather than legislation - Thanks for being such a jackass, Mitch!) since LBJ, and from the moment he left office Trump and his cronies have been trying to roll all of that back and then some.
Biden has some of the luster from Obama, and has in the eyes of many proven that he will return the loyalty black voters are showing to him by having acted as if he believes loyalty goes both ways. It's something no other person who was running had, probably not even the actual black candidates.