We don't have a bull (Stag/Vixen) but we DO have black men as friends. She meets alone with only one of our black friends. They work at the same hospital and they meet after work, so usually he just gets a blow job. I think my wife is unique because she often has an orgasm when he ejaculates. When the three of us meet I usually watch for a little while, but then I join on the fun.
We've done mostly MFM, but lately have talked about trying to arrange a MFMM. That has been a bit difficult so we haven't been able to get one going.
We've been doing this for almost five years and don't expect to stop any time soon. Truth is, MFM with black men has become a very large part of our sex life.