black flag vs Jolly Roger

black flag and Jolly Roger both are nouns.

Word	Noun	Adjective	Verb	Adverb
black flag	Yes	No	No	No
Jolly Roger	Yes	No	No	No

As nouns, black flag and Jolly Roger are synonyms defined as:

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

Other synonyms of black flag include blackjack, pirate flag.

black flag (noun)	Jolly Roger (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