Dark patches around my joints—is this normal?

My issue is that my skin is darker around my joints. My hands are what worry me most because my fingers end up looking black-and-white-black-and-white 🙂
It isn’t a massive difference, just about one shade darker than the rest
The worst part is during the summer; once I tan, it becomes obvious. This is just how my skin has always been. I have an olive undertone, so I don't burn, I just get dark... I know someone else dealing with this exact same thing. Does anyone know why skin gets darker at the joints, or how to mask it?