The Collaborative International Dictionary of English v.0.48:

Blue grass \Blue" grass`\ (Bot.)
   A species of grass (Poa compressa) with bluish green stems,
   valuable in thin gravelly soils; wire grass.
   [1913 Webster]

   Kentucky blue grass, a species of grass (Poa pratensis)
      which has running rootstocks and spreads rapidly. It is
      valuable as a pasture grass, as it endures both winter and
      drought better than other kinds, and is very nutritious.
      [1913 Webster]

WordNet (r) 3.0 (2006):

blue grass
    n 1: any of various grasses of the genus Poa [syn: bluegrass,
         blue grass]