The Collaborative International Dictionary of English v.0.48:

Blackboard \Black"board`\ (bl[a^]k"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. In
   late 20th century similar boards of a green slate as well as
   some colored white became common; wrioting on the slate
   bioards may be done with chalk, but writing on the white
   boards is done with colored pens, such as grease pens, which
   leaves a trace that can be easily erased. The newer boards,
   usualy called chalkboards are nevertheless still sometimes
   referred to as blackboards.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blackboard
    n 1: sheet of slate; for writing with chalk [syn: blackboard,
         chalkboard]