Dark tea is fully oxidized like a black tea, and then fermented. Many in the tea field call the oxidation of black teas "fermentation," which is a fallacy. Fermented tea has (usually) been oxidized, then is either left ot its own devices to ferment naturally (called Sheng or Raw) or 'fast-fermented' with an existing bacterial culture (called Shu, Ripe, or Cooked).
The sheng dark teas tend to be more mellow and vegetal in flavor with earthy undertones, as they take significantly longer to ferment (typically seven years until it's ready to drink, many argue up to twenty years until it's "done"). The shu dark teas, on the other hand, come out much darker and richer due to their quicker fermentation, turning incredibly dark and earthy much more quickly.
It was traditionally pressed into cakes for transportation, but we have it loose as well. It is one of the oldest types of tea in China with a rich history of over 1700 years that can be traced back to the Eastern Han Dynasty (25-220 AD).
The most popular type of dark tea is called Pu-Erh, named for the mountains in Yunnan, China in which the style was perfected. The name pu-erh is only to be used for dark teas from the pu-erh mountains, but is commonly attached to any dark tea (much like champagne or tequila). Both of the varieties of dark tea we stock are shu pu-erhs (one loose, one brick), though we hope to not only stock sheng pu-erhs in the future, but dark teas from other sources as well.