blister vs pustule

blister and pustule both are nouns.

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

As nouns, pustule is a hyponym of blister; that is, pustule is a word with a more specific, narrower meaning than blister:

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

Other hyponyms of blister include water blister, blood blister.

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