Black olives might in fact be green olives dyed black.
The color has nothing to do with the variety of olives. Green ones were picked unripe and change color from reddish brown to black as they ripen.
Unripe olives are firm and mild in taste, whereas black ones are more bitter.
Whether ripe or not, freshly picked olives are extremely bitter and need to be brined for months in order to become edible.
Some producers, however, dye green olives with artificial coloring, such as ferrous gluconate. This is not illegal, but needs to be clearly displayed on the product declaration.
Whether an olive is dyed or not can be seen from the pit. Black olives have black pits.