bulla vs blood blister

bulla and blood blister both are nouns.

Word	Noun	Adjective	Verb	Adverb
bulla	Yes	No	No	No
blood blister	Yes	No	No	No

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

bulla: (pathology) an elevation of the skin filled with serous fluid
blood blister: blister containing blood or bloody serum usually caused by an injury

Other hyponyms of bulla include water blister, pustule.

bulla (noun)	blood blister (noun)
the round leaden seal affixed to a papal bull	blister containing blood or bloody serum usually caused by an injury
(pathology) an elevation of the skin filled with serous fluid