darken vs overcast

darken is not a noun while overcast is a noun.

Word	Noun	Adjective	Verb	Adverb
darken	No	No	Yes	No
overcast	Yes	Yes	Yes	No

As verbs, overcast is a hyponym of darken; that is, overcast is a word with a more specific, narrower meaning than darken:

Other hyponyms of darken include embrown, murk, dun, blind, dim, dim, bedim, benight, shade, shade off, shadow, cloud.

darken (noun)	overcast (noun)
	a cast that falls beyond the intended spot
	a long whipstitch or overhand stitch overlying an edge to prevent raveling
	the state of the sky when it is covered by clouds
	gloomy semidarkness caused by cloud cover

darken (adjective)	overcast (adjective)
	filled or abounding with clouds

darken (verb)	overcast (verb)
become dark or darker	sew with an overcast stitch from one section to the next
make dark or darker	sew over the edge of with long slanting wide stitches
tarnish or stain	make overcast or cloudy