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]

The Collaborative International Dictionary of English v.0.48:

blackwash \black"wash\ (bl[a^]k"w[o^]sh), v.
   1. same as defame.
      [WordNet 1.5]

   2. to color with blackwash.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

blackwash
    n 1: an attack intended to ruin someone's reputation [syn:
         character assassination, assassination, blackwash]
    2: a mixture of calomel and limewater that is used on syphilitic
       sores [syn: blackwash, black lotion]
    3: a wash that colors a surface black
    v 1: bring (information) out of concealment
    2: color with blackwash