Trump / the Sleepy Doctor '16.
That's the only way I can see him recovering at least a little bit. Trump maybe didn't go out of his way to alienate them like he did with Hispanics, but the white supremacist stuff and his supporters beating up black protestors on tv really hurts any chances at doing better than Romney did.
On the other hand, there is not a whole lot to suggest that Republican African American candidates do any better than non-AA Republican candidates (iirc, anyhow). On top of that, Carson's completely moronic statements may hurt his appeal somewhat if weaponized into ad campaigns.