To answer your question from my perspective, I would say that at least 75 percent are curious. They have heard the rumors and want to know if they are true or not.
That being said, I would say that somewhere in the vicinity of less than 50 percent have tried it at least once - with the overwhelming majority having done so while single.
As far as those that are 'active' I would say about 5 to 10 percent - at the most.
But, all that being outlined, 'Normal' couples are actually quite 'Abnormal' While the wife may not be interested in Blacks, per se, the majority are in to some sort of 'abnormal' behavior behind closed doors - may that be BDSM, Female Dom, or any of the other activities I'm sure you've all read about. But they put on a 'normal' face to the public.
For example, I've been active for about four years now. I know, beyond a doubt, no one suspects my interest. We always meet at different hotels, never establishing a pattern and I don't confide with any of my friends. As far as they are concerned, I lead a very normal, humdrum life.
When you find a way to crack into their brains, I think my assumptions outlined above will prove to be generally pretty accurate.
Donna