Black tea is oxidized the longest to in comparison to green tea.
Black tea originated from China and it is through trade that black tea spread to neighboring countries. Black tea became popular because, on the trade route, black tea retains its flavor better than green tea. Although green tea and black tea come from the same plant, it is the production process that differentiates them.
One of the more popular types of tea today, English Breakfast and Earl Grey, are all a blend of black tea. Black tea is more popular in the West in comparison to green tea in China and Japan.