We don't have synonyms for nigrescent yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are ignescent, vitrescent and virescent.
(n) :
the figure of the moon as it appears in its first or last quarter, with concave and convex edges terminating in points; something shaped like a crescent, especially:
(adj) :crescent-shaped: Anything having the shape of a crescent or new moon; marked by an increase, Waxing, as the moon;
(n) :
the figure of the moon as it appears in its first or last quarter, with concave and convex edges terminating in points; something shaped like a crescent, especially:
(adj) :crescent-shaped: Anything having the shape of a crescent or new moon; marked by an increase, Waxing, as the moon;