My current GF does that, sucks the skin too, does the Angela White tongue swirl too, I make sure when she does something I like to tell her that’s hot and moan a bit. I’ve had quite a few girlfriends in the past who did that, I had one girl blow it up like a balloon, that took some training. As long as you make it fun, positive, and never criticize, most women are cool doing all sorts of stuff. It’s a real skill to let a girl know she can express her sexuality without judgement or criticism with you, when she feels comfortable doing it you’re in for a real good sex life.