black grass

Webster's Dictionary (1913)

Botany, a grasslike rush of the species Juncus Gerardi, growing on salt marshes, and making good hay.

From the entry Black.

a kind of small rush (Juncus Gerardi), growing in salt marshes, used for making salt hay.

From the entry Grass.

Webster’s Revised Unabridged Dictionary (1913), from GCIDE 0.54