Black blood blisters on the lower legs which are painless and itchy may be related to trauma and small skin vasculitis. 1. trauma: if the leg is squeezed and other trauma, it can cause capillary rupture and black blood blisters appear without pain or itching. 2. Small skin vasculitis: vasculitis that simply involves small dermal blood vessels, mostly occurring in the lower legs, can be manifested as erythema, papules, purpura, blisters, blisters, vesicles, vesicles, ulcers, necrosis and superficial nodules. Black blood blisters may be the blood blisters rupture bruising, but also can have pain and itching is not obvious. If there are black blood blisters on the lower legs and do not itch, it is recommended to go to the hospital in a timely manner, to clarify the cause of the disease, under the guidance of the doctor for standardized treatment.