A blood blister in the anus may be caused by the following conditions. You may have thrombosed external hemorrhoids. Most of them are caused by anal subcutaneous capillary rupture resulting in bleeding, should be absorbed but due to the anal part of the special, after the bleeding can not be fully absorbed so the formation of thrombus, this thrombosed hemorrhoids will cause discomfort in the anal area, sometimes it will be a severe pain. Thrombosed external hemorrhoids is more serious need to timely treatment, in addition to the perianal abscess will also appear this symptom, which is mostly due to the formation of abscess after the blockage of anal glands, the attack is accompanied by swelling and pain, the situation is not serious can be conservative treatment of medication, the ineffective or pus need to be incised and drained. In addition, there may be sinusitis, anal polyp attacks and other conditions caused by the best can go to the hospital to do a perianal examination.