Part of this flavor profitle is due to the fact that these teas are highly oxidized, around 80% or more. This gives it the melow and rich flavors its often associated with.

The process black tea goes through starts off with Withering to help release a large amount of the 70% water content of each leaf. Once an appropriate amount of water has been shed, the tea is put into a rolling process to gently bruise the leaves and start their oxidation process. This is repeated a few times until the leaves are evenly bruised and starting to show signs of oxidizing. From here, they get left out to undergo the majority of their oxidation, reaching the desired level of the processor. At this point, they go through a drying process where hot air is blown over the leaves until the water content in the leaves reaches around 5%. From this stage, the tea is now ready to be sorted and packaged.

China

Although it is now a very popular option throughout the world, black tea had very humble beginnings. Green tea was the only type produced until the Ming dynasty in China (mid 1300s-mid 1600s), where the story tells of farmers interrupted from their tea making as a traveling militia came through the Wuyi mountains. Because of this, they had to leave out their leaves to oxidize far longer than intended. In an attempt to save their tea harvest, they decided to smoke it with pine wood, and found that they loved the flavor of this almost discarded tea. Black tea has spread ever since, becoming more and more popular as tea spread throughout the west for its easier storability for long journeys across the world. 

Britain

Part of the reason for Black tea becoming the most popular has to do with the British. While they controlled a colony in India, they discovered that tea is the product of one specific plant, Camellia sinensis, and a variant grew natively in the Indian Assam region - Camellia sinensis var. assamica. This meant that they no longer had to expensively import tea made in China when they could just grow their own. They quickly discovered that this new variety of the plant had not been adapted to tea as well as the Chinese version which had been bread specifically for making tea for thousands of years, however. To rectify this, they stole trimmings from the Chinese variety, Camellia sinensis var. sinensis, and made a hybrid version with the Assam variety. This hybrid is now the most grown cultivar in the world.