Random realization (not super recent but still fairly recent)—black olives usually sold in cans in USA are only black because of the brine they are in. The black color comes from an iron rich ingredient in the brine. I had no idea. I lived most of my life without knowing this.
(too tired to look up a link again right now, but if you look you can find one)
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@JeremyMallin I don't think that's entirely true. According to the international olive oil council, "black olives" are fully-ripe olives. It could be that the brining process artificially ripens them, but it doesn't appear to be the direct reason for the blackening.
@JeremyMallin Link for that source here: https://www.internationaloliveoil.org/wp-content/uploads/2019/11/COI-OT-NC1-2004-Eng.pdf