cul de sac vs blind alley

cul de sac and blind alley both are nouns.

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

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

Other synonyms of cul de sac include dead-end street, impasse.

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