Have you told her or talked to her about it yet? Is she being reluctant, if so make sure you have plenty of IR porn around and are watching it EVERYTIME you have sex! Tell her how hot you think it is and let her know that if she ever wanted to try it that you are in TOTAL support and will be there for her!!!!
Then start withholding sex, soon she'll be wanting fucked and hopefully during that time you are telling her to go black so when she decides she's not waiting for you to fuck her she'll be looking for a black guy!
I have "trained" myself to go however long it takes. My wife always turns to a black guy when she isn't getting it at home and often times anymore she gives it up whenever she wants more black cock. She knows I approve so long as she knows they are clean and they don't hurt her.