The Collaborative International Dictionary of English v.0.48:

Black wash \Black" wash`\ (bl[a^]k"w[o^]sh) or Blackwash
\Black"wash\, n.
   1. (Med.) A lotion made by mixing calomel and lime water.
      [1913 Webster]

   2. A wash that blackens, as opposed to whitewash; hence,
      figuratively, calumny.
      [1913 Webster]

            To remove as far as he can the modern layers of
            black wash, and let the man himself, fair or foul,
            be seen.                              --C. Kingsley.
      [1913 Webster]