Source : Webster's Revised Unabridged Dictionary (1913)

Blackboard \Black"board`\ (-b[=o]rd`), n.
   A broad board painted black, or any black surface on which
   writing, drawing, or the working of mathematical problems can
   be done with chalk or crayons. It is much used in schools.

Source : WordNet®

blackboard
     n : sheet of slate; for writing with chalk [syn: {chalkboard}]