Hi. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concerns.. As per your complain a painful blister on gum following injury with toothbrush bristles is a GINGIVAL ABSCESS or a GUM ABSCESS which can occur due to bacteria invading the gums following the injury caused by toothbrush leading to formation of a pus filled sac along with inflammation leading to formation of blister and pain.. You should consult a dentist and get evaluated and an x ray can be done to evaluate that whether the abscess is limited superficially to the gums or has invaded deeper into the supporting tissues of tooth leading to formation of a Periodontal abscess.. In case of superficial infection limited to the gums probing the blister to drain the pus by topical application of a local anaesthetic gel or spray and draining out the pus along with localized scaling of the gums in that area followed by antibiotics like Amoxicillin and Metronidazole and painkillers like Ibuprofen for relief.. Gargles with Chlorohexidine mouthwash can be done.. In case if it has invaded deep causing Periodontal abscess then deep cleaning of the gums in that area and curettage followed by antibiotics and painkillers will help.. Hope this information helps.. If you find the answer helpful please give me a Five Star Review and click on found the answer helpful as a token of appreciation.. Thanks and regards.. Dr.Honey Nandwani Arora..