I believe that some ethnic groups are prone to hyper-pigmentation and the anal area happens to be an area that often time get's darker that surrounding areas. I think some people may feel that it is off-putting to be dark there for the obvious reason. They may feel that in their opinion it looks dirty. I actually think it's beautiful because it's natural. I've noticed that mostly darker skinned Latino, Italian, and Asian men tend to get this. I think there is more of a desire to have an even skin tone that matches the rest of their body. I am darker complexioned black American but I oddly have the opposite going on where the area around my hole is much lighter. It's never been an issue for me and only one partner has made a comment about it but it was a comment that expressed he liked it. As I recall he pulled my cheeks apart and told me it looked like a pussy which at the time made me self conscious but then he assured me he thought it was very inviting to him as he also happened to be bisexual and thoroughly enjoyed pussy. There is no procedure that I know of that would make the skin around your sphincter darker.