pirate flag vs black flag

pirate flag and black flag both are nouns.

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

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

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

Other synonyms of pirate flag include blackjack, Jolly Roger.

pirate flag (noun)	black 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