My wife and I were in this same situation before we had our first threesome with a BBC a few years ago and I got her a realistic chocolate skin dildo that was 8.5 inches with girth for Christmas. We used it for about 3 months before one day I walked in on her riding on top of it mounted to a chair. I just jokingly said “Once you go Black.”
We found a nice respectful hung black guy after 2 months of searching and had a threesome in a hotel room. We were young and it was our first time sharing, we thought it would be just a one time thing and it was for a couple of years but now I’m getting urges to share her again since we’re getting older and I’m way more comfortable in my marriage.
To your predicament id suggest surprising her with a realistic dildo like I did, don’t hesitate to spend a little more on a good quality one. My wife still uses hers years after our “one time” BBC experience, so I knew she would def be open to it again. I’ve recently asked her about it and she said yes to another threesome and now we’re looking for the right guy. Just go with her reaction to the dildo, whether she is just receptive to the fantasy or would even go all the way