darken vs black out

darken and black out both are verbs.

Word	Noun	Adjective	Verb	Adverb
darken	No	No	Yes	No
black out	No	No	Yes	No

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

Other hyponyms of darken include dusk, blacken out, cloud over, cloud up, overcloud.

darken (verb)	black out (verb)
become dark or darker	lose consciousness due to a sudden trauma, for example
make dark or darker	suppress by censorship as for political reasons
tarnish or stain	darken completely