blue grama vs grama grass

blue grama and grama grass both are nouns.

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

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

blue grama: a pasture grass of western North America
grama grass: pasture grass of plains of South America and western North America

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

blue grama (noun)	grama grass (noun)
a pasture grass of western North America	pasture grass of plains of South America and western North America