The Collaborative International Dictionary of English v.0.48:

Coalsack \Coal"sack`\, n. [Coal + 2d sack.] (Astron.)
   Any one of the spaces in the Milky Way which are very black,
   owing to the nearly complete absence of stars; esp., the
   large space near the Southern Cross sometimes called the
   Black Magellanic Cloud.
   [Webster 1913 Suppl.]