My wife isn't specifically into black guys, but she loves the big Shane Diesel dildo. She doesn't cum on the dildos, because she prefers saving that for the real thing. Her choice, on that. She has never, since I have known her used one without me around. My wife definitely has messed with black guys in the past. She has had a single mfm marathon with two black guys as well. Many creampies.
I think it turns her on more from my reaction to her experiences, than from her actual experiences. I get massively turned on from a lot of things I haven't done, myself, but fantasize about. It's hard to say, really, so don't let it drive you crazy. In time, without pushing, obsessing, you'll probably know from her own mouth, if that's true.
You can always play a game where you tell her something she didn't know about you, and she tells you something you didn't know about her. Could learn a lot, even if it isn't specifically that one detail.
One thing is for certain, when my wife gets horny, it seems like she's almost up for anything. I love that about her.