the percentage of Black ppl in the scene really depends on the communities (kink/fetish) and the city... the ones that are into BDSM are usually in spaces with Cacs and quite few of them are on some

especially when you factor in ethnicity and social status... no different from the escort/call girl/companion world.
ATL/Houston/New Orleans are the best places for us southern brehs.... PNW can be the worst place for brehs if they are only into Black women but the PNW/San Fran still have hands down the best spots to get into whatever kink you like and to see if you want to go deeper into the lifestyle... Also some of the best kink/fetish/BDSM masters resides on the westcoast if you are trying to learn more about the craft/lifestyle (art of spanking/choking, shibari rope tying, also BDSM aftercare - which is the most important thing you will have to learn as a Dom) it will be more easier and cheaper to find them in that area especially if you are planning to attend a workshop.
As for Black couples you see them on the scene from time to time but most are not swingers, they just looking for inspirations and new toys to introduce to the bedroom, or take some workshop classes on BDSM Basics.... the ones who are swingers usually was already into the lifestyle on their own and they met in the community and just became a couple.