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]