The Right loves unqualified candidates period, not just unqualified black ones. Many trace the genesis of this to Sarah Palin, but I think it actually goes back to Dan Quayle. Donald Trump, Christine O'Donnell, Todd Akin, Michele Bachmann and Rick Perry are not black.
And Dr. Carson is a horrible presidential candidate. Just the fact alone that he publicly said he wants to abolish Medicare and Medicaid shows a level of stupidity, both politically and as a matter of policy, which should automatically remove him from contention. The GOP "establishment" knows this is a general election deal breaker, but apparently the base is too lost in a Fox News fantasia to see reality.
Quayle and Perry at least held elected office. O' Donnell-Nah she got lucky in the hard-right year in 2010. Akin-The McCaskill Campaign basically won the primary for him and I think just hearing since that time from McCaskill she knew there was a chance that Akin would slip up and make a mistake on the campaign trail which he did do.