Lewis & Short

dē-nī̆gro, āre, v. a., to blacken utterly, dye or color very black (very rare; in the Ciceron. per. not at all).

  1. I. Lit.: terram (amurca), Varr. R. R. 1, 55 fin.: lanam, Plin. 33, 6, 35, § 109: capillum, id. 23, 5, 53, § 99: faciem super carbones, Vulg. Thren. 4, 8.
  2. * II. Trop.: honorem famamque alicujus, to blacken, i. e. to asperse, defame, Firm. 5, 10 fin.