I'm about in the same boat as you. I've long fantasized about having my thick blonde wife taken and controlled by a Black Bull. My wife knows I want to see her with a Black Man and be submissive, but is hesitant. She didn't say no though. More worried about borrowing trouble with the wrong person. I share her concerns. After I'm convinced that I have a suitable Bull, I'm thinking of setting up an initial "inadvertent" meeting. My first idea is at a local adult store, which we've talked about going to together. The Bull could establish contact and hit on her there. The other approach is more direct, set up the meeting with the Bull with my wife totally in on the deal. Probably the safest approach, but maybe she doesn't get up the courage to go through with it? She's into the adult store journey, so this might be a good way to break the ice.
I'd be interested in people's ideas. Each has its advantages and risks. Biggest potential problem is if the wife finds out about the setup and reacts badly. You need to be sure she's ready to go through with it. In my case, she wouldn't be pissed as she's contemplating already. I just need to give her the final push.