black grama vs gramma

black grama and gramma both are nouns.

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

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

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

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

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