The Collaborative International Dictionary of English v.0.48:

blackwater \black"wa*ter\ n.
   any of several human or animal diseases characterized by dark
   urine resulting from rapid breakdown of red blood cells; --
   used especially of

   blackwater fever, a severe form of malaria caused by the
      blood parasite Plasmodium falciparum.
      [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

blackwater fever
    n 1: severe and often fatal malaria characterized by kidney
         damage resulting in dark urine