The Collaborative International Dictionary of English v.0.48:

Blackheart \Black"heart`\, n.
   A heart-shaped cherry with a very dark-colored skin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blackheart
    n 1: any of various diseases in which the central tissues
         blacken
    2: heart cherry with dark flesh and skin cherry [syn:
       blackheart, blackheart cherry]