The darkening of the arm bend may be caused by the following reasons: 1. It may be caused by genetic factors, some due to genetic factors, there will be congenital abnormal pigmentation in the corner of the joint, for example, some patients will appear in the armpit, groin or popliteal fossa and elbow fossa of the elbow joint and other parts of the above pigmentation, and then lead to the darkening of the situation. 2. This is the result of subcutaneous bleeding, such as in this area due to trauma or puncture injury, resulting in subcutaneous bleeding in this area, due to changes in hemoglobin, it is possible to produce blackening of this area. 3, may be the result of inflammatory hyperpigmentation here, resulting in this condition has many causes, such as trauma or by the aseptic inflammation here. The process of receding inflammation may lead to some excessive pigmentation, which in turn causes darkening here. Similar pathological changes may also occur if there is an infected lesion.