Stacey Abrams would be good too. That said, he's not really struggling with the black vote. He's going to struggle with the progressive wing, that's why I think Liz might be a better strategic move. Plus, she could probably deeply influence the direction his administration would go in considering she has actual plans for things.
EDIT: He'll also struggle with Hispanic and Latinx voters due to Obama's deportations. So, maybe even Julian Castro?