black music vs R and B

black music and R and B both are nouns.

Word	Noun	Adjective	Verb	Adverb
black music	Yes	No	No	No
R and B	Yes	No	No	No

As nouns, R and B is a hyponym of black music; that is, R and B is a word with a more specific, narrower meaning than black music:

black music: music created by African-American musicians; early forms were songs that had a melodic line and a strong rhythmic beat with repeated choruses
R and B: a combination of blues and jazz that was developed in the United States by Black musicians; an important precursor of rock 'n' roll

Other hyponyms of black music include blues, soul, hip-hop, rap, rap music, rhythm and blues.

black music (noun)	R and B (noun)
music created by African-American musicians; early forms were songs that had a melodic line and a strong rhythmic beat with repeated choruses	a combination of blues and jazz that was developed in the United States by Black musicians; an important precursor of rock 'n' roll