I think Kamala Harris has already been chosen. She's a mixed bag. She won't help with black men because she's married to a white man, but because she's bi-racial & a woman, she's a white liberal's wet dream so she'll appeal to the general women base. She won't help him in the electoral college, but she'll body Pence in the VP debate so that will be fun to watch. She's attractive too so that helps.
No other black woman comes close. Biden isn't picking a mayor, a Congresswoman or a former State Rep. Susan Rice would be demonized just as bad as Hilary Clinton was by the right wing media. Amy Clinton is boring as hell and won't add excitement to his ticket.