The possible causes of blood blisters on the body are as follows: 1. In most cases, they are formed by a certain amount of bleeding on the basal surface of the blister, which is most likely herpes zoster or chicken pox. In the case of herpes zoster, the main manifestation is a blister on the basis of limited skin erythema, and the blister continues to develop due to a certain amount of bleeding on the basal surface, so the formation of blood blisters. In addition, there are also patients who have chickenpox, especially adult chickenpox, if there is inflammatory bleeding on the basal surface, will also produce blood blisters; 2. If it occurs in both lower extremities, it may also be allergic skin vasculitis in the area of both lower extremities. Allergic cutaneous vasculitis can also produce a hemorrhagic reaction in the subcutaneous tissues if the inflammatory reaction is obvious; 3. Allergic purpura can also produce symptoms of hemorrhagic blisters if the inflammatory reaction is obvious when formed.