Getting one's spouse into BBC or any sexual fantasy involving others, requires a lot of preliminary issues be addressed first, and if she's up tight about sex, in general, this often requires a lot of issues being addressed plus time involement. To start with, this is YOUR fantasy, probably not her's, and SHE is taking all of the physical risks with her body, not you.
Also, as sure as you seem to be on wanting to see your wife taken by another man, particularly a black man, the more insecure you might be once you see her not only enjoying it, but requiring it, the less likely you'll want it to continue long term. No longer would YOU be in control. No longer would it be YOU deciding when she had sex with another man. You would become secondary to her desires and needs, and I do believe most men (including myself) would not desire a wife who fucked other men against my desires or wishes. Unintended emotional attachments & egos often pop up, unplanned, and the otherwise sound marriage gets challenged Big Time ... if children are in the home, they are the first to suffer under the arguments and bitterness that suddenly begin to develop. So, its important that one be able to separate their FANTASY from their REALITY ... as one is real, the other not so.
Your profile information doesn't indicate any experiences with other men inside the marriage, and you haven't volunteered any here. Hope you'll offer some background info if you truly desire sincere responses. .......Mac