darken vs dun

darken is not a noun while dun is a noun.

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

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

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

darken (noun)	dun (noun)
	horse of a dull brownish grey color
	a color or pigment varying around a light grey-brown color

darken (adjective)	dun (adjective)
	of a dull greyish brown to brownish grey color

darken (verb)	dun (verb)
become dark or darker	make a dun color
make dark or darker	cure by salting
tarnish or stain	persistently ask for overdue payment
	treat cruelly