Among the main quality attributes of black olives, colour is the most organoleptically important. A less mature green olive can be blackened either naturally (developed by maturity) or artificially (developed by oxidation). The natural blackening of olives is mainly attributed to formation and accumulation of flavanols, flavones and anthocyanins during maturation. Olives can be darkened by different methods, some of which utilize chemicals and even dyes in some locations. So the darkening of olives applied by small processors is a hazardous process due to the use of artificial dyes and/or low grade commercial chemicals contaminated with heavy metals. This study summarizes the applications for darkening during olive processing.