Purple and black skin may be poor peripheral circulation, subcutaneous bruising, trauma and other reasons, as to whether necrosis will occur, should be determined on a case-by-case basis, can not be generalized. 1. Poor peripheral circulation: If the peripheral circulation is poor due to strenuous exercise, anoxic environment, cold or emotional excitement, the skin will turn purple and black. In such cases, it is necessary for the doctor to determine the cause of the attack and treat the symptoms. 2. Subcutaneous stasis: Increased vascular fragility of the skin and organs or a significant decrease in platelet count can lead to subcutaneous stasis, which is manifested by purple and black skin, petechiae, ecchymosis and so on. 3. Trauma: due to the injury of external forces on the skin tissue, caused by the bleeding and edema of the subcutaneous mucosa, appearing as purple and black skin. The above three cases can not be directly judged whether necrosis phenomenon, if the symptoms of purple and black skin, it is recommended to consult a regular hospital in a timely manner, under the guidance of the doctor, to take the appropriate treatment program, so as not to delay the condition.