blind alley vs cul de sac

blind alley and cul de sac both are nouns.

Word	Noun	Adjective	Verb	Adverb
blind alley	Yes	No	No	No
cul de sac	Yes	No	No	No

As nouns, blind alley and cul de sac are synonyms defined as:

Other synonyms of blind alley include dead-end street, impasse.

blind alley (noun)	cul de sac (noun)
(figurative) a course of action that is unproductive and offers no hope of improvement	a street with only one way in or out
a street with only one way in or out	a passage with access only at one end