grama vs black grama

grama and black grama both are nouns.

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

As nouns, black grama is a hyponym of grama; that is, black grama is a word with a more specific, narrower meaning than grama:

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

Other hyponyms of grama include blue grama, Bouteloua gracilis, Bouteloua eriopoda.

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