疱 is the medical term - it appears in compounds like 疱疹 (herpes), 水疱 (blister), 脓疱 (pustule). The 疒 radical signals 'illness/affliction'. In casual speech, blisters are usually just called 泡 (also pào), which 疱 is sometimes treated as a more clinical variant of.
Outer illness radical - the indexing radical, picturing a person lying on a bed with a covering above. Anchors 疱 in the disease/symptom family with 病 sick, 疼 ache, 疯 mad, 疫 plague. Marks the entry as a medical/skin-condition word.
Inside 包 supplies the sound bāo → pào with consonant softening and tone shift. Also reinforces meaning: a blister is a fluid-wrapped pouch on the skin - a tiny 包 of liquid. Same phonetic family: 抱 hug, 泡 bubble, 跑 run, 饱 full. The 'rounded enclosed sac' image makes the etymology vivid.
