Paint blistering looks exactly as it sounds. Paint begins to peel and puff up from the surface, forming small to large surface bumps that look like blisters. This occurs when painted walls or ceilings have a constant exposure to high humidity, which is why it's such a common problem in bathrooms. Luckily, once you address the humidity, you can fix the blistered paint without fear that it will reoccur.
Open the windows and door after you shower, or install a fan in the bathroom. If you don't address the humidity, your paint will constantly blister and peel.
Place a disposable dropcloth on the floor of your bathroom. Open all the doors and windows. Scrape the blistered paint off from the walls with a paint scraper.
Sand the area with 100-grit sandpaper and dust with a feather duster. Apply mildew-resistant primer with a roller or paintbrush. Allow the primer to dry completely.
Apply one coat of mildew-resistant paint to the scraped areas with your roller or brush. Allow the paint to dry and apply another coat. Allow the paint to dry at least 24 hours before showering in your bathroom, ideally longer.