How to tell the difference between black zinc (or any zinc plating) and phosphate:

Dip a toothpick in a fairly strong acid and deposit a small drop on an unexposed part of the test piece. I use phosphoric acid (Rust Cure) for this, because I always have some, and I know how it behaves on zinc. If tell tale bubbles soon appear, the coating is likely zinc, because zinc is highly reactive with acid. Apply water to stop the reaction, and no harm done.

Mostly you never want to put zinc plated parts in acid, because the zinc will be totally consumed in a fraction of an hour or so, even with a weak acid like vinegar. Zinc plating is usually in the range of only 0.0001" to 0.0002" thick. When doing zinc plating or chromating, HCl is used for a dip cleaner, but the exposure is in seconds.