Blood filled blister on roof of mouth...Suddenly appears and suddenly burst spilling blood into the mouth. It is day 5 and lesion is still there?

Have it looked at. There are many types and causes of blisters. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections and insect bites can cause blisters on the skin. Have it evaluated by a professional and get the appropriate treatment.
Mucocoel. You may have had a mucocoel on the roof of your mouth which ruptured an began to bleed because of trauma. You should see a dentist or oral surgeon to also rule out a fistula from an abscessed tooth. The mucocoels are benign.
See a dentist ASAP. That blister is either from an infected tooth that is developed an abscess that was gone untreated, or a soft tissue lesion that need to be examined and treated properly.