How can I get mrs wife involved ?

I have not actually broke this fantasy to my wife or have done in the past when takes and My Mrs has just dismissed it as me joking.

But I really want to see her with another guy preferably a black guy .

how can I break it to her ?

I would say your best bet would try it while not takes. It will probably have much more impact. Try first as pillow talk than after you do that for a while talk about it outside of the bedroom.

I agree with this approach. Bring up the subject several times, but not in a joking mood. Have a semi excited manner and approach it as something that would bring excitment into your marriage. When she's in a more playful mood, ask her "If we were ever to do such a thing, what would you like the other guy to be like?"
If she answers, you're half way there.