Darkness in the middle of the butt crack may be related to local thickening of keratin, pigmentation and folliculitis. 1. Local thickening of keratin: especially for people who have been sitting for a long time or always riding a bicycle, because the skin in the middle of the buttock crack is subjected to friction for a long time, which leads to local thickening of keratin and roughness of the skin, so it will become black. 2. Pigmentation: due to improper diet, long-term sedentary and long time without cleaning and other reasons lead to the middle of the buttock crack skin cells secrete too much melanin, so that the local pigmentation, can appear in the middle of the buttock crack black phenomenon. 3. Folliculitis: If the skin in the middle of the butt crack has folliculitis, the local skin is ulcerated and inflamed. And because this place is easy to be friction, not easy to breathe, when the skin damage is more serious, may lead to local skin black. If the middle of the butt crack black, to go to the hospital in time to find out the reason, so as not to cause adverse consequences.