The back is blue for no reason, which may be related to traumatic collision, hemangioma, purpura or petechiae, coagulation dysfunction and so on. 1. Traumatic collision: if the back area is hit by external force without attention, it may lead to localized injury of soft tissues, causing subcutaneous ecchymosis, which looks like a greenish-purple patch from the outside, and may be accompanied by pain and swelling. 2. Hemangioma: If the patient has hemangioma, it may be manifested as skin elevation, light blue patches of skin, congestive patches of skin and so on. 3. Purpura: If the patient has immune thrombocytopenia, it may cause purpura or even petechiae, and the patient will have a blue patch of skin. 4. Coagulation dysfunction: If the patient has abnormal coagulation function or lack of coagulation factors, there may be subcutaneous bleeding phenomenon, which will be manifested as bruises on the skin. It is recommended that patients seek medical treatment in a timely manner to do systematic examination to clarify the causes of the disease.