The Collaborative International Dictionary of English v.0.48:

Black snake \Black" snake`\ (sn[=a]k) or Blacksnake
\Black"snake\, n. (Zool.)
   A snake of a black color, of which two species are common in
   the United States, the Bascanium constrictor, or racer,
   sometimes six feet long, and the Scotophis Alleghaniensis,
   seven or eight feet long.
   [1913 Webster]

   Note: The name is also applied to various other black
         serpents, as Natrix atra of Jamaica.
         [1913 Webster]

WordNet (r) 3.0 (2006):

blacksnake
    n 1: large harmless shiny black North American snake [syn:
         black rat snake, blacksnake, pilot blacksnake,
         mountain blacksnake, Elaphe obsoleta]
    2: blackish racer of the eastern United States that grows to six
       feet [syn: blacksnake, black racer, Coluber
       constrictor]