Hi! Welcome to Healthcaremagic. I read your query. This blister on gum corelates with periapical infection. The tooth with crown is Root canal treated or not? If not, then it should be done. Also, reinfection can develop in already root canal treated root due to secondary infection, etc. X-ray at different angulations, percussion test of tooth can help in establishing the infected tooth. However, if it is not happening, clearing infection from root area can also be done. This is called as apoecectomy. It will require cutting through bone, clearing infection, retrograde root canal of tooth and stitches for a week. The procedure is done under local anesthesia and antibiotics and painkiller take care of postoperative infection and pain. However, in case of RCT treated tooth infection, that needs to be cleaned thoroughly too, which is not quite easy with apoecectomy. I suggest you visit an oral surgeon and get diagnosis done accurately and then treatment. Hope the answer helps you. Thank you!