Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concerns.. As per your complain blood blister in mouth after physical trauma like as you explained caused by getting pinched with a bridge can be due to rupture of a small blood vessel under skin.. Nothing to worry about it as it is mostly self limiting and resolve on its own.. As of now you can take symptomatic treatment to relive pain and discomfort like applying a numbing gel and a steroid ointment over it to relieve pain and inflammation.. Do cold compresses over it.. You can also take Tylenol for pain.. Take multivitamin supplement, avoid hot and spicy food and maintain a good oral hygiene like by gargling with antimicrobial mouthwash.. In case if it does not start improving in a week consult an Oral Physician and get evaluated.. Hope this information helps.. Thanks and regards. Dr. Honey Nandwani Arora.