Here's the old Cain thread
Sklansky thought 33% for Cain was "high but close" versus the first black President, so I suppose 27% is a backtrack.
That might be reasonable for a moderate Republican, but I think Carson's Tea Party positions would hurt him quite a bit among this overwhelmingly liberal demographic. Also, Obama would almost certainly be endorsing his successor and that would count for a fair amount.
Still, some otherwise conservative, weak black Republican candidates have done relatively well among blacks in various state elections according to exit polls. For instance, Kenneth Blackwell, the OH SoS who came under fire for voting machine irregularities in the 2004 election that were believed to have disproportionately affected blacks, still won 20% of the black vote despite losing by over 20 points overall. (Source in that thread somewhere).
My guesstimate is ~15%.