The Collaborative International Dictionary of English v.0.48:

Bluebottle \Blue"bot`tle\, blue-bottle \blue-bottle\, n.
   1. (Bot.) an annual Eurasian plant (Centaurea cyanus) which
      grows in grain fields; -- called also bachelor's button.
      It receives its name from its blue bottle-shaped flowers.
      Varieties cultivated in North America have showy heads of
      blue or purple or pink or white flowers

   Syn: cornflower, bachelor's button.
        [1913 Webster + WordNet 1.5]

   2. (Zool.) A large and troublesome species of blowfly (Musca
      vomitoria). Its body is steel blue.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bluebottle
    n 1: an annual Eurasian plant cultivated in North America having
         showy heads of blue or purple or pink or white flowers
         [syn: cornflower, bachelor's button, bluebottle,
         Centaurea cyanus]
    2: blowfly with iridescent blue body; makes a loud buzzing noise
       in flight [syn: bluebottle, Calliphora vicina]