Canned black olives are actually just chemically-treated green olives
Like tomatoes, olives change color as they ripen. They start off green and then turn red then purple and finally black. However, when you buy black olives, they haven't naturally ripened. Instead, they're picked when they are green and cured with dilute brine and lye solutions.
This causes the olives to oxidize and turn black. Thankfully, despite what some might expect, they aren't dyed black with chemicals but instead oxidized to turn a uniform black color.
