The Collaborative International Dictionary of English v.0.48:

black fly \black fly\, blackfly \blackfly\, (Zool.)
   1. In the United States, a small, venomous, two-winged fly of
      the genus Simulium of several species, exceedingly
      abundant and troublesome in the northern forests; --
      called also buffalo gnat. The larv[ae] are aquatic. It
      sucks the blood of birds as well as humans and other
      mammals.
      [WordNet 1.5 + WordNet 1.5]

   2. A black plant louse, as the bean aphis (Aphis fab[ae]),
      which infests e. g. beans and sugar beets; -- called also
      bean aphid.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

blackfly
    n 1: blackish aphid that infests e.g. beans and sugar beets
         [syn: blackfly, bean aphid, Aphis fabae]
    2: small blackish stout-bodied biting fly having aquatic larvae;
       sucks the blood of birds as well as humans and other mammals
       [syn: blackfly, black fly, buffalo gnat]