not that I'm the best one to give advice - since my wife hast tried it yet... but she HAS come a looong way, from borderline racist to the sweet little white housewife who only cums REAL hard when she got "black dick on the brain", role-plays, and really gets off letting black men see her body.
My advice to you... dont rush or pressure her - make it about what SHE wants... and making sure you're catering to THAT so she dont feel self conscious about her taboo, sluttier side.
Of course there may be suggestions for a quick-fix.. in that case, I'm all ears too!