What is Black Tea?
Black tea is especially popular in India and Turkey, but there are still many black teas of excellent quality in China. The process always includes the four basic steps, namely the painting, rolling, kneading, oxidizing and firing. The lensing of the leaves causes them to become limp. After this, the leaves are rolled and kneaded so that the juices and enzymes are released from the cells. Afterwards, the tea is oxidized and not only gets a dark color but also a different taste.
Finally, the tea is dried or fired to stop oxidation.

What makes black tea different from green tea is that during the production process the tea leaves are allowed to oxidize completely before being heat processed and dried. During oxidation, oxygen interacts with the cell walls of the tea plant to give the leaves the rich dark brown to black color that black tea leaves are famous for. Oxidation also changes the flavor profile of a black tea, helping to add malty, fruity or even smoky notes, depending on the tea.

In contrast, when green tea leaves are processed, they are minimally oxidized. After they are harvested, they are quickly heated and dried to prevent too much oxidation that would turn the green leaves brown and alter their freshly picked taste. Less oxidation means green tea tends to be lighter in color and flavor than black tea,

Caffeine content in black tea
Between coffee, black tea and green tea, coffee generally has the most caffeine content per cup, then black tea, followed by green tea. But as with any beverage brewed from a caffeinated plant, there are many factors that can determine the caffeine content in your cup of black tea, including how the plant was processed and how the beverage was brewed.