blood blister vs bulla

blood blister and bulla both are nouns.

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

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

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

Other hypernyms of blood blister include bleb, blister.

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