Unfortunately, there is not much you can do. If you keep bringing it up, she will feel pressured and will probably become more opposed to it.
As access to your profile is blocked, I can't see where you are located, assuming you bothered to say, so I no idea if this is feasible, but if you have jazz and/or blues clubs in you area and you have an interest in such music, you might start taking her there to hang out. Jazz and Blues venues tend to attract a fair amount of black people, especially musicians.
You might get lucky if she meets some men and/or sees any she finds attractive. Other than that, you would need to do some research to see if there are any other venues that draw a mixed racial/ethnic crowd in your vicinity where you can take her so that she might start thinking in the direction you hope for.
Have you tried getting her to do any kind of "roll play" sexually? Maybe by playing out roles where you become other men to her might reset her thinking about being with other men as an OK thing. Try a simple scenario like being a cop that is arresting her: interrogate her then put her against the wall and frisk her. Of course you will find a reason to do a strip search and body cavity check as well. If you have any kind of uniform looking shirt or other props then its even better.
You can think of other "characters" or even ask her if she 's ever fantasized about any particular guys like a cop, fireman, mailman, repairman, etc. and then find some props and build a fantasy for her.
The idea is to see if you can loosen her inhibitions up and make her more receptive to the idea that enjoying sex can be fun and not only "making love".
No matter what though, keep in mind that most women are emotionally and sexually monogamous and it is only a small percentage that can actually separate sex from love. Your wife may not be one of them.
So while this may be a disappointment to your sexual fantasies, if you truly love your wife, be happy that she values you more than getting some strange dick.