is it possible to say it like "Az új autók feketék?"
Is there any difference in emphasis between the two?
It's mostly about what you want to get across first. "Feketék az új autók?" may suggest you don't feel much need to clarify you are talking about "the new cars" while "Az új autók feketék?" starts with clarifying what we are even talking about.
Why is új not in the plural form?
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).
I think that would be: "Új autók a feketék?" (Are the black ones new cars?) An answer could be: "Nem, a feketék régi autók." (No, the black ones are old cars.)