The most common factor is melanin deposition, because the buttocks are not easily exposed to direct sunlight and are a private part of the body, so they do not receive sufficient sunlight, resulting in local melanin deposition and deposition. 2. The pressure on the buttocks is the greatest among all organs and organs of the body, because people often adopt a sitting posture, and the buttocks are under the greatest pressure when they are sitting. This is because excessive local pressure can also lead to skin discoloration and pigmentation changes, so blackening can occur. 3. Pathological conditions, such as hyperadrenocorticism and other factors can lead to excessive melanin deposition, which can also lead to blackening of the buttocks in this pathological condition.