black flag vs pirate flag

black flag and pirate flag both are nouns.

Word	Noun	Adjective	Verb	Adverb
black flag	Yes	No	No	No
pirate flag	Yes	No	No	No

As nouns, black flag and pirate flag are synonyms defined as:

black flag and pirate flag: a flag usually bearing a white skull and crossbones on a black background; indicates a pirate ship

Other synonyms of black flag include blackjack, Jolly Roger.

black flag (noun)	pirate flag (noun)
a flag usually bearing a white skull and crossbones on a black background; indicates a pirate ship	a flag usually bearing a white skull and crossbones on a black background; indicates a pirate ship