White blister in the mouth? hurts only a little bit. what could it be?

Oral Apthous Ulcer: Canker sores (apthous ulcers) are shallow erosions with a white center surrounded by a ring of pink to red inflammation. They can be minimally painful to severely painful. They usually take about 1 week to heal. If they do not heal, have it checked out. There are many causes for blisters/erosions in the oral cavity and differentiating the cause is important for long term treatment decisions.

Aphthous Ulcer: An aphthous ulcer called canker sore is most likely. There are other things, so you should see a dentist for a diagnose and treatment.