There are several clinical reasons for the blackening of the knee: a. The patient’s accidental fall during the activity can cause skin contusion at the knee accompanied by local bruising, which is manifested as local skin color blackening, and the patient has swelling and pain. You can improve the blood circulation of the injured tissues through adequate rest, local ice packs within 24-48 hours, and hot packs after 48 hours to promote faster absorption of the bruises and reduce the performance of the knee color blackening. Second, the patient’s knee receives too much ultraviolet radiation, can form local pigmentation manifested as black color, which is a normal physiological phenomenon. Most patients can gradually alleviate the symptoms of black color after avoiding continued UV exposure. Third, the occlusion of arterial vessels at the knee can cause local tissue ischemia and hypoxia, resulting in local skin color blackening, decreased skin temperature, and even tissue necrosis, which can be clarified by arterial angiography or ultrasound examination of the nature of the lesion.