Olives naturally turn black as they ripen. When unripe they are green. As they ripen they get reddish, then purplish and finally black.

"Ripe Black Olives" in a can are actually olives which are neither black nor ripe when they are picked. They are picked very green and then cured using dilute brine and lye solutions. Lye treatments cause natural phenolic compounds in the olives to oxidize to a black color. Calcium chloride salts, iron salts (ferrous gluconate) and compressed air bubbled through the curing vats help develop the black color. So there is no black dye used but the olives are treated to make them a nice uniform dark black.

Home curing of olives without these added salts will result in mottled and brownish olives which taste just as good but are not as attractive.

The opposite tact is used to make a nice green California- style green-ripe olive. Air containing oxygen must be excluded to avoid natural oxidation and darkening of the skins.

For a great discussion on the California ripe olive industry, see Louise Ferguson's chapter in the Olive Production Manual put out by the University of California.