Dark skin between piercing

So I got my belly piercing a little over 4 months ago and I just started to notice the darkening of my skin. It’s not itchy nor does it irritate me. I also clean it religiously and have no problems with it whatsoever except for the darkening of the skin, is this normal? And what can I do to get rid of the darkness?

(the jewelry is long and made of plastic btw because I had a problem while back and my piercer suggested to get the original jewelry replaced with that for a while so I can bend properly and so that there’ll be more space for the piercing to heal)