What to do if you have a dark circle around your nostril piercing?

I would say its getting infected. I had a nostril piercing and it gets infected, get a wet towel but it has to be hot and put it on there almost every day ........trust me it works

The chances are good that the nostril screw in your piercing is made of Silver and not Surgical Stainless Steel. Silver oxidizes and this tarnishing will actually darken the tissue around the piercing. Changing the jewellery to proper surgical stainless steel body jewellery will resolve the tarnishing issue. Over time with daily washing the darkness around the piercing will wear off and you should be fine.