my soft palate developed "a blister full of blood" that then popped and now have a very sore place in the roof of mouth.  This is the 2nd time in a year and a half.  The last time I went to my dentist each day for him to monitor and we never could determine the cause. and just out of nowhere it happened again today.  Very sudden onset....like it all occurs within 5 minutes.
Thanks for your query, I have gone through your query. The blood blister can be secondary to trauma while tooth brushing, or it can be secondary to burns after Chat Doctor.  Nothing to be panic, consult a good oral physician and get it evaluated. Meanwhile you can take topical anesthetic like lidocaine and topical analgesic like choline salicylate. Do saline gargling. I hope my answer will help you, take care.