Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
nī̆gresco, grŭi, 3, v. inch. n. (usu. ī) [id.], to become black, grow dark (poet. and post-Aug. prose): latices nigrescere sacros, etc., Verg. A. 4, 454: tenebris nigrescere, id. ib. 11, 824: nigrescunt sanguine venae, Ov. A. A. 3, 503: baca incipiens nigrescere, Plin. 15, 1, 2, § 6: nigrescens cutis, id. 26, 1, 5, § 7: nigrescentes dentes, id. 31, 10, 46, § 117: bacae cum jam nigruerint, Col. 12, 48: dies nigrescet, Vulg. Ezech. 30, 18.