black grama vs grama grass

black grama and grama grass both are nouns.

Word	Noun	Adjective	Verb	Adverb
black grama	Yes	No	No	No
grama grass	Yes	No	No	No

As nouns, grama grass is a hypernym of black grama; that is, grama grass is a word with a broader meaning than black grama:

black grama: a pasture grass (especially of western coastal regions of North America)
grama grass: pasture grass of plains of South America and western North America

Other hypernyms of black grama include grama, gramma, gramma grass.

black grama (noun)	grama grass (noun)
a pasture grass (especially of western coastal regions of North America)	pasture grass of plains of South America and western North America