I wanted to answer your question but I'm backing out of the thread since I made the point I wanted to make.
Do I think it's ok to ask a black person if you can touch their hair? Of course. Would I personally? Not unless I was already their close friend and had a feeling that answer would be yes. But questions are fine to ask. And the person being asked can say no and that should be respected. This goes for anyone.
Fun story, my mom was born in Japan and she had bright blonde hair. The people there were in awe of a blonde child and touched her hair all the time. I don't think they asked first. Not that it really matters. It was just sort of relevant to the conversation we're having now.