At this point, I think electability arguments based on race are a wash. I can't imagine African-Americans voting for Hillary in large numbers if they feel her candidacy lacks legitimacy.

Can she really carry Ohio, Pennsylvania or Michigan if African-Americans don't vote for her in large numbers? And forget about making Virginia blue or retrieving Florida.
There are no easy choices for the Democrats for calibrating the race issue. Yes, many white people won't vote for Obama because he's black - we don't need studies to tell us this. And, many African-Americans will sit out the general election if Hillary is the nominee - again, no studies needed.