I would start by continuing to encourage her inner hot wife for awhile if she has never gone Black before. As she has sex with more and more other men, introduce some interracial porn featuring Black men (no gang bangs at first just one on one action). Toss any other porn you have a few at a time while bringing in more IR. Get her enthralled with Black cock sliding into white pussy, and always get her off while she watches IR porn. After awhile let is slip that you would love to see a Black cock sliding in and out of her pussy.
Keep planting seeds, sooner or later something will grow . . .