Yes, and it seems to act as a singular negative according to the accepted answer which is "Apples are not blue" Vs. "No Apples are not blue". Is that right?
Yup. If we were translating word for word, it would be "No apples are not blue", but the Slavic languages often feature double negatives not only meaning a single negative, but actually required for the sentence to be properly negated (but only once :D).
Could one say "Żadne jabłko nie jest niebieskie"? When does one use plural vs singular with żaden?
I guess you should keep to the grammatical number here. The meaning is virtually the same, but technically your sentence is "Żadne jabłko nie jest niebieskie".
Sure, but you could take as well as criterion to preserve the number of negations, or reflexives, or whatever, but that would be bad idea. Thing is, when you translate you lose structure and you cannot ask here to preserve number. And anyway is "no apples are blue" correct English?