ni·gres·cence 
(nī-grĕsəns)
(nī-grĕsəns)n.
1. The process of becoming black or dark.
2. Blackness or darkness, as of complexion.
[From nigrescent, blackish, from Latin nigrēscēns, nigrēscent-, present participle of nigrēscere, to become black, from niger, nigr-, black; see nekw-t- in the Appendix of Indo-European roots.]
ni·grescent adj.