Blisters can be caused if you apply paint to warm surfaces such as those heated by direct sunlight. The paint dries too quickly to form a film before the vapors get a chance to escape. Moreover, the radiating heat expands the air, forming bubbles.
Blisters are also caused if you apply an alkyd or oil-based paint over a surface that has not dried up thoroughly. Wood can retain moisture within its grains, and therefore when paint is applied there is lack of adhesion in certain areas, causing the film to lift off and form blisters.
Although latex-based paints rarely form blisters, they too can have problems adhering to surfaces with high humidity, resulting in blisters.
With time, some blisters may decrease in size and get leveled on their own. If they don’t, repair leakages and deal with the sources of moisture. If your walls are getting moist due to outside elements such as rain or dew, or some kind of a leakage, get this fixed before painting the walls. Install exhausts and vents in kitchen and bathrooms.
You must then scrape the blisters off gently, sand the area, apply a good primer and then apply a coat of a good acrylic paint. Preparation is the key. Always sand and prime the walls before painting.