Since tea as discovered at 2700s BC, China, which some leaves accidentally dropped into a glass of hot water, or so the story goes, and tea brewing as we know it was born.
As the ancient Chinese began to produce more and more tea, they realized that with a special fermentation process, tea leaves became darker. The tea made from these leaves was more potent, and the leaves could be stored for longer periods of time without losing their potency. These tea types were called black tea because of the change in coloration of the leaves through the fermentation process.
The fermentation in black tea brewing is really a process that oxidizes the tea leaves: it also increases the amount of caffeine by weight, making black tea a unique energy source.