Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
nī̆gror, ōris, m. [1. niger], blackness (poet. and in post-Aug. prose): noctis, Pac. ap. Cic. Div. 1, 14, 24; Lucil. ap. Non. 515, 5: mortis, Lucr. 3, 39: nigrorem in ulceribus excitat, Cels. 2, 1: rubidus est rufus, atrore et nigrore multo mixtus, Gell. 2, 26, 14.