Totally agree with all the comments above that a honest, direct conversation is the way to go. There should be no tricking or pressuring -- that's weak, a major turn-off, and is the one thing most likely to ruin the possibility. Even if the answer ends up being "no", a woman will at least respect honesty, and it helps build the trust, communication, and understanding which are all good things for your relationship whether it leads to sharing or not.
But, I'll add one more reason to be straightforward, which might be controversial... A woman's mind is often a swirl of thoughts, feelings, and anxieties. Don't get me wrong - I'm a strong woman, a feminist, and I know women have many strengths that men don't. But taking decisive action, especially when risk is involved, is not always one of them -- no matter how *big* (and black) the payoff might be.
Most women have already thought about fucking a black man, but may have fears or barriers (some conscious, some *******) that prevent us from exploring those desires. Three common worries I hear are:
1. If I do this, my hubby/bf will think I'm a slut and not love me anymore. He says he wants this, and that he'll still love me afterward, but how can I be sure he will?
2. I wonder if he's just testing my loyalty. If I say yes, I'm worried I will fail his test and he will dump me.
3. I'm worried that he is only proposing this because he wants to cheat, and will use this as an excuse to fuck other women. I don't want that.
If a boyfriend/husband can communicate his desires clearly and unapologetically, it can help put these fears to rest. That's why I often encourage men to be confident, honest and straightforward about what they want, while also making it clear that it's fine if a woman isn't ready.
Here's some language I've suggested men use: "One of my biggest turn-ons is imagining you with a hung black stud. I would love to see you experiencing that kind of extreme pleasure. And, I think it would bring us closer together -- you'd be fulfilling a fantasy of mine, and we’d share a very naughty, intimate experience. Many couples do it. But if you don't want to do it, I understand, and we don't have to. You should also know: My fantasy is to see you with a black man, or as many black men as you desire, but I would never want to be with any other woman except you. I love you and I'm devoted to you and your pleasure." I'm assuming these are all true statements for the man, of course.
I'll just speak for myself, and not "all women" here: I am attracted to decisive, confident, assertive men in part because I know they can get me out of my head and over any inhibitions that keep me from adventure and pleasure. Nothing dries my pussy out more than a man who is hesitant, overly apologetic, or worse - so afraid of my opinions that he tries to trick me or indirectly persuade me. I'll take an arrogant douche-bag over a pussy any day.