What Causes The Formation Of Black Ring Around The Anus?

I am white btw. I have a dark purple/blue/blackish ring around my anus. It has been there for years (maybe it was always there?) and I have not had any problems with it. I know for a fact that I m not talking about Hemmerheoids and there is no itching. I also know that it is incredibly common for people to have dark anuses, but simply the bruise like color of it concerns me. The only potential cause I can this of is that I use anal toys fairly regularly (one a week at most), but I don t think that that would leave a mark like this