When olives remain on their tree for long enough, they ripen and their colour changes from green to dark reddish brown or purplish black. Leave them for too long and they will turn soft and decay looms. You have to harvest black olives at precisely the right time then.
Most olive farmers - and even those from Cyprus - have devised a trick to circumvent that problem: they harvest the olives when they are still firm and green. And then dye them black. Green olives are immersed in a solution of ferrous gluconate (E579), which oxidizes them. That generates a uniform black colour, even darker then they ever would ever become if they would remain on their tree.
Ferrous gluconate is water soluble iron salt of gluconic acid. Gluconic acid occurs naturally in some fruits and vegetables, wine and honey.
Ferrous gluconate is also an effective medication in the treatment of hypochromic anemia. It gives your iron levels a much needed boost, which means that black olives may be even more healthy than previously thought.
Did I already mention that green olives also get a little make-over too? Their fresh green colour is the result of a bath with a green pigment chlorophyl (E140), commercially extracted from nettles, grass and alfalfa.
The taste of olive oil harmonizes excellently with the fragrance of Mediterranean herbs. In the Mediterranean countries, olive oil is often flavoured with branches of rosemary, lavender, tarragon or, on Cyprus, with fresh capers. Most fresh herbs can be preserved in olive oil; their aroma compounds dissolve better in oil than in an aqueous medium. A most famous recipe of this kind is pesto, a paste of ground basil leaves in olive oil.