A blister on your labia majora is most likely due to a clogged pore or follicle. Conditions such as ingrown hairs (folliculitis barbae) or “razor bumps” (pseudo-folliculitis barbae) are common if patients shave or wax their pubic region and often result in local inflammation or sometimes pain.
Treatment consists of cleansing the area, applying warm compresses, and applying salicylic acid toners to the affected area.
If you are sexually active, blisters in your pubic region can also be secondary to STDs such as those caused by HPV, Herpes Virus, Syphillis, or Chancroid (Haemophilus ducreyi). HPV is the cause of genital warts which results in irritated skin that can sometimes present as a blister. Herpes Virus is another STD that causes painful blisters to appear in affected areas. A painless ulcer in the genital region that bleeds can be the first sign of a Syphillis infection. Finally, chancroid infections result in a painful ulcer in the genital region.
It is impossible to diagnose your condition without a physical exam and tests. Therefore, a visit to your gynecologist is warranted to rule out any potential for STDs.