The darkening of the butt crack pinch may be a normal physiological phenomenon, an allergic reaction or other factors. 1. Physiological phenomenon. Darkening of the butt crack may be a normal physiological phenomenon. Individual differences may be accompanied by darkening of the butt crack, generally do not need special treatment. 2. Allergic reaction. The darkening of the butt crack, accompanied by redness, swelling and itching, is considered to be related to an allergic reaction. Skin allergies may cause an inflammatory reaction, post-inflammatory hyperpigmentation, resulting in local black color. Long-term sedentary, friction degree is large, resulting in local pigmentation, there will be black color. Butt crack black, if there is no any discomfort symptoms, it is recommended to closely observe. If there are various discomfort manifestations, such as itching and pain, it is recommended to go to the hospital for a clear diagnosis.