It depends on what she is comfortable with now. Is she into swinging already? Then all you have to do is go to a swinging party with a black guy and half the battle is won. If she is not into swinging, is she into watching porn with you. Swinger and interracial porn are great ice breakers. The "screw my wife" series is awesome.
However you go, there are hurdles that you have to overcome.
1) Girls have been programed to hide/stiffle their sexuality. All women initially resist the idea of any extramarital sex because she thinks she has to in order to be "proper".
2) When the husband suggests swinging the first thing the girl thinks is "he just wants to fuck other chicks". She has to know that you just as, if not more interested in her being satisfied too.
3) A good way to break the ice is post-fucking pillow talk. She is satisfied, she knows you love her, and she won't be as intimidated by certain questions. Good ones to ask include:
"If you could have sex with any celebrity, no consequences, who would it be? (Later, ask about people you know, or people she or you work with)"
"Did you have sex with anyone before we met? Who with? What was it like for you?"
"Who was the biggest guy you were with? What was it like?
If you are generally curious, and even aroused by her answers, she will become more comfortable talking to you about it. If you have jealousy pangs when she starts talking about fucking your best friend, or her ex boyfriend who was so much bigger than you, then you need to back off. Not for her sake, for yours.
Once she gets into a story about a time she was really satisfied by another man, then reply "That's kinda hot". She'll ask you to tell her who you would like to fuck. Reply "I don't know, I think you are so sexy, I love being with you so much, you are so erotic when we make love, It would be incredible to be able to watch you have sex, not participate, just watch." You are not telling her you want her to fuck your friend, or even a black guy. You are not saying that you want to watch her get fucked, you are saying nothing more than you would like to be able to experience sex with her from a different perspective.
Regardless, you don't come off as a sick perv, and she starts thinking about the idea of her fucking another guy.
Another thing. If she ever gets hit on by anyone. Get interested. Ask her what she thought, how she felt. What she thought of the guy. Make sure she knows that you are not and would not be upset by her having thoughts about another guy. Tell her that you love her and trust her implicitly. Tell her that you are not threatened at all by her being attracted to another guy. If she responds positively, then move in and start to seduce her. If you can fuck her right then, then during sex, bring up the other guy. Ask her if she wants to be fucked by him. If you have played the situation right, and if she is turned on, she will say yes. Afterwards, don't say anything about it, and don't bring it up for a while. Let the idea stew and grow.