blind alley has 3 distinct synonyms spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a street with only one way in or out
(figurative) a course of action that is unproductive and offers no hope of improvement
“all the clues led the police into blind alleys”
Synonyms, antonyms and the broader and narrower relations for blind alley come from Princeton University’s WordNet. Full sources and licences.