An adjective is pluralized only if it's the predicate of a sentence (like "feketék" here). In other words, never pluralize an adjective if it's directly in front of a noun.
Could it not also mean are the blacks ones new cars? (as opposed to old cars or maybe telephone boxes)
Judit, i can't be 'the black ones' because the word 'the' (a/az) is not in front of the word 'black' (feketék).