A blister on the outside of the anus may be caused by external hemorrhoids, anal herpes, anal eczema and other factors. 1. External hemorrhoids: may be related to bad habits, bad bowel habits, heredity and other factors, can lead to blisters on the outside of the anus, may also be accompanied by itching, pain and other symptoms. 2. Anal herpes: When herpes zoster virus infection exists in the anal area, it can also cause blisters to appear in the anal area, accompanied by itching as well as tingling, which is contagious. 3. Anal eczema: may be caused by chronic infection, food factors, environmental factors, can make the anus localized red pimples, similar to blisters, accompanied by intense itching and tingling. If there is a blister on the outside of the anus and there are other abnormal manifestations, it is recommended to consult a doctor for examination and treatment to avoid causing other complications.