The purple bruises on the arm may be caused by the following reasons: 1. It may be a local hemangioma, which is usually an abnormal proliferation of blood vessels in the veins, causing the color of the black bruises to recede when pressed, and the local bruises can be restored immediately after releasing the pressure. 2, It may be caused by local soft tissue contusions or pressure or crush injuries, which have a clear history of trauma. 3. It may be caused by coagulation dysfunction, such as congenital thrombocytopenia or lack of coagulation factors. 4. It may be caused by patients who have been taking oral anticoagulants for a long time due to heart disease, and it is easy for local subcutaneous capillaries to rupture and bleed.