Denigro, demgras, pen. cor. denigrâre. Plin. To make blacke.
Lewis and Short: Latin dictionary
dē-nīgro, āre, v. a., to blacken utterly, dye or color very black (very rare; in the Ciceron. per. not at all). 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.— II.Trop.: honorem famamque alicujus, to blacken, i. e. to asperse, defame, Firm. 5, 10 fin.