What may cause a blood blister on palate?

Trauma. Usually it is the result of trauma such as rough food (taco chip?), or suction. But if it doesn't disappear within a week -10 days, call your dentist to check it. If it doesn't resolve it might be something more serious. Please follow up on this.
Trauma. Usually some type of trauma or injury. If it does not start to heal in a few days, you need to see a dentist to do an exam and determine if something else is going on, .
Various causes. Trauma cam cause a blood blister on the palate, or it could be related to an infected tooth, just appearing like a blood blister. Best to see a dentist to have it evaluated.