pustule vs blister

pustule and blister both are nouns.

Word	Noun	Adjective	Verb	Adverb
pustule	Yes	No	No	No
blister	Yes	No	Yes	No

As nouns, blister is a hypernym of pustule; that is, blister is a word with a broader meaning than pustule:

pustule: a small inflamed elevation of skin containing pus; a blister filled with pus
blister: (pathology) an elevation of the skin filled with serous fluid

Other hypernyms of pustule include bleb, bulla, hickey, pimple, zit.

pustule (noun)	blister (noun)
a small inflamed elevation of skin containing pus; a blister filled with pus	(pathology) an elevation of the skin filled with serous fluid
	(botany) a swelling on a plant similar to that on the skin
	a flaw on a surface resulting when an applied substance does not adhere (as an air bubble in a coat of paint)