Hah! Just got the straight dope (This is actually not where I read this before. It's just the first link that came up this time.)
Black olives in cans are picked before they're ripe, then artificially ripened; exposure to oxygen makes them turn black. They have to be cooked in the can to inhibit bacterial growth. Green olives are picked before they ripen and are pickled in brine which has the proper pH to inhibit bacterial growth without the need for cooking.
Ripe olives are either purple or black. So, if you're getting un-canned black or purple olives, that means that they were allowed to ripen on the tree. They're still packed in brine, though.
Far more than I ever wanted to know about olives. Just sayin.