Sounds like she is ready but YOU'RE not
She was DOING it, and you freaked. She picked out a guy and started to get it on with him, IN PUBLIC! I don't think you have any idea how RARE and spectacular that is. She took ownership of her sexuality. Damn, she IS hot. BLESS her! I will toast her tonight!
From what I can discern, the issue here is not how to get her to go black, but how to get her to go black within your very limited terms. I would caution you that you may not be able to remain completely in control of events and passions once they are released. Have you talked to her about why you got pissed off? Are you only willing to have her do guys you choose, only when and where you choose them? How does she feel about that? What are HER terms? If she likes the idea of being used, but your concept doesn't work, you need to find out why. How can you let her have what she needs and still get what you need?
It would work for me if my wife were submissive. I could get great pleasure from offering her to a guy or guys of my choosing, completely disregarding her tastes and wishes. Just so happens she is far more dominant than submissive, and that kind of approach simply makes her angry and resentful. It doesn't work, and anything vaguely obligating her to perform sexually backfires so badly NOBODY gets any. But she IS willing to make her own choices. I love that she is out there dating, and she is being real. She is not a cock-tease, and she is becoming more comfortable fucking on a first date. Me? I love that she has begun to celebrate and exercise her sexuality. Women such as yours and mine are far too wonderful creatures to be restricted to just one man. It's kind of a crime against mankind to do so. So I'm delighted if she is getting dick, from me or whoever, as often as possible. She will never be completely indiscriminate, but she is terrifically adventurous, and getting better every week. I believe a woman is more beautiful the more numerous and diverse her sexual partners. Yours needs to be out there, too.
You need to make it a partnership. Engage her in the process. Discuss how you two could put together a scenario which would work for both of you. You say that if she blew up you wouldn't force her, and she has demonstrated a willingness to do it, so you just need to work out the HOW. Propose it as a fantasy enactment - a role-play. Ask her how it would go for her, if it fulfilled her most beloved fantasy. Find out what pushes her buttons, and figure out how to give her that experience without trashing your own. Tell her what makes it most exciting for you. We voyeurs and clean-up guys are getting a lot of our sex vicariously through our wives, and we NEED to enlist them in the process as partners. If they are eager participants, even in a submissive, abused role, it will work far better for us. When they get more of what they want, we get more, too. It can be win-win.
I understand not doing the cuckold thing. I kind of like my wife to behave dismissive or arrogant toward me in a sexual context, like making me drive her to a date and wait for her while she is getting laid, but I would not play the cuckold to one of her guys AT ALL. How about if your lady had first come to you that night and begged your permission? Would she have to kneel and kiss your zipper? How about if she had asked you to remove her panties to demonstrate that permission? How about if she has to promise to come right back to you, with her pussy full, to prove that she has been a good girl for you? What if you obligated her to pick out a guy and get him to fuck her in the men's room? Maybe she would owe you one of your choosing for every one of her own. If she is truly submissive, maybe make it ten for one!
Seems to me there are lots of ways to work it out, and YOU DON'T NEED TO DUPE HER. You don't have the issue of her guilt, which can sometimes only be surmounted by doing it TO her. You are way better off than that. SHE IS WAY MORE READY! You just scared the piss out of her, and you owe her an apology. I believe she thought she was doing EXACTLY what you wanted. Brought him over to show you, scared and excited but still proud. You smashed that precious and delicate confidence that most women NEVER acquire. Grovel if you have to, Dude, but GET THAT BACK AGAIN! IT'S WORTH MORE THAN GOLD!
I'd be honored to broker the negotiations, if you thought it might help, or open correspondence with her to help out in any way. You're already a winner, with a great-looking gal, and a player to boot. Best of luck, amigo!