I'm black and I think I know exactly what you're talking about. I'm assuming you mean the skin feels normal and isn't a different texture, just coloured darker as opposed to rougher in texture or leathery or something, right? I haven't ever given it a lot of thought, maybe because I think it's very common in us more pigmented people. I've always thought/been told that it's caused by friction in damp areas e.g. thighs rubbing together, upper arms rubbing against the body- that sort of thing. Maybe because it's soooo common except maybe in really slim people - the sort with thigh gaps and an appreciable distance between their arms and bodies, I've never given it a second thought. It's only now that I think about it, that I'm realizing that maybe it's not common with caucasian people. Although, I have seen lots of caucasian people with more tan/red/brown/darker skin in the friction areas than the rest of their bodies. I think it's normal as long as it's not sudden onset or leathery or tough or extra itchy or anything.
One thing I can say is that at 275 lbs, the darker skin on the insides of my thighs extended a lot further than it does now. Now that there's a little less friction, the skin has faded a lot and is only dark at the plumpest parts of my thighs near the inside tops of my thighs where my thighs still rub quite a bit.



