Having darker skin around the anus, and in the perineum in general, is very common. If you are really selfconscious about it, there are some techniques that can be used. The following information is taken from Wikipedia:
Anal bleaching is the process of lightening the color of the skin around the anus. It is done for cosmetic purposes, to make the anus more uniform with the surrounding area. Some treatments are applied in an office by a technician and others are sold as cream that can be applied in the privacy of one's own home.