The neck of the foot is blue, considering that the ankle joint may have subcutaneous bleeding, common trauma or spontaneous bleeding, and may be caused by venous blood vessel lesions in the ankle. 1. Trauma: when the patient’s ankle area is traumatized or sprained, resulting in damage to the subcutaneous tissue and muscle, small blood vessels rupture, which can cause subcutaneous hemorrhage, and the ankle joint may appear swollen, bruised and blue. 2. Spontaneous bleeding: such as allergic purpura, anaphylactic purpura, etc., the patient’s coagulation function decreases, capillary permeability increases, the affected red blood cells seep out of the blood vessels, resulting in subcutaneous hemorrhage, which may appear in the ankle joints to form a patchy greenish-purple color. 3. venous blood vessel lesions: when the patient ankle venous blood vessel lesions, such as cavernous hemangioma, varicose veins, the neck of the foot will also appear the phenomenon of blue. There may be other reasons for the blue ankles, it is recommended that patients consult the hospital in time to clarify the cause of the disease for targeted treatment.