Have you guys talked to your wives about what HER fantasy is? Do you know which actors she finds sexy? Maybe she's not attracted to black men in particular. Maybe it's more about the way a man acts, or his accent, or dresses, or a thousand other things than the way he looks that gets her excited. Maybe she'd like to be with a woman, or to watch you with a man. Perhaps she would like to dominate or be dominated in a safe manner. Until you have this level of honesty, trust, and safety to communicate, introducing the idea of another person being involved in your sex life will likely not go well.
Your fantasies are valid. So are hers. Find out what hers are and work with those as a start. Maybe she hasn't thought much about it and formed a clear concept that she can communicate. Or maybe she's not comfortable enough to communicate it or even allow herself to think about it. Help her to explore without trying to steer her. Trust us, giving your partner their fantasy is very hot for both of you. forsing your fantasy on to them is not for either. Once you find out her fantasies you can share yours with her. You both don't have to like each other's fantasies, but you can try to find ways to safely incorporate them.
This next part you need to be sincerely willing to do without any expectation that they will reciprocate. But this should be easy because you'll be helping them to experience their fantasy in a controlled manner.
The two of you should dedicate an evening to be all about her and the idea of her fantasy. This night won't necessarily be about acting out the fantasy, but having her experience it in her mind. This may need to start well before any sex happens. Plan the evening ahead of time with aspects of her fantasy. Don't go overboard with it and take your cues from how she reacts to the subtle aspects and build from there. DON'T interject your fantasy! This night is about her.
Once in bed, whisperer a scenario involving her fantasy in her ear while you pleasure her. Use your hands and tongue much more than your cock, if you use it at all. Have toys ready if they are appropriate for her fantasy. Give her more orgasms than you ever have before. Don't stop until she stops you. Continue by holding and kissing her. Snuggle for awhile for her to re-energize. Then suggest you go again. Repeat as much as she can all night long. You don't need to cum until she is done (or maybe not at all). If she feels obligated to service you, tell her this is her night and she can watch as you take care of yourself (or perhaps, if you can take it, say this is her night and all the orgasms are hers).
The next day be sure to ask, if she doesn't bring it up, how she liked her fantasy. This isn't the time to ask for her to reciprocate with your fantasy. Have sex several more times, sometimes incorporating some of her fantasy to a lesser degree, maybe with both of you orgasming while you are talking about it.
After this it would be a good time to ask if she would be willing to tell you a story involving your fantasy.
Hope this helps.