My wife is picky too. We had someone we had known for a while and she liked his personality. Without him, I doubt she would have done it. She still has never slept with a black man who was a stranger, because she has to know and like a man or she's just not comfortable with it. That's her boundary that we still have not crossed. She has gotten to know a few of here bull's black friends intimately, but that was with the permission of her bull, not her own doing. If your wife is like mine, the key if for her to get to know some black men socially and find one she "likes". You'll know it's good when she laughs and smiles around him. My wife seemed to look into his eyes a bit too long and stand a bit too close to him while talking. She also might wind up playing with her hair while talking to him. These wound up being a signs of attraction she didn't really notice or admit to. But, when he became the fantasy sexual focus for her, she became very lubricated talking about him.
Bottom line: E x p o s e her to some available black men. Get them over to play poker, or watch a football game. Don't force it. See how she acts around them. Does she talk much more easily around one particular man? How long do her eyes meet his? Don't pick a man for her and try to get him to seduce her. Too risky if he fails. Let her choose and that man will become her black bull and he won't be playing poker at your kitchen table, he'll be playing "poke her" in you marital bed.