Source : Webster's Revised Unabridged Dictionary (1913)

Black wash \Black" wash`\or Blackwash \Black"wash\, n.
   1. (Med.) A lotion made by mixing calomel and lime water.

   2. A wash that blackens, as opposed to {whitewash}; hence,
      figuratively, calumny.

            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.