Black tea is more oxidized than the green, oolong and white varieties; all four varieties are made from leaves of Camellia sinensis. Black tea is generally stronger in flavor and contains more caffeine than the less oxidized teas. In Chinese black tea is known as red tea; a more accurate description of the color of the liquid. In Chinese, "black tea" is a commonly used classification for post-fermented teas, such as Pu-erh tea. However, in the western world, "red tea" more commonly refers to South African rooibos tisane.
Generally a teaspoon of black tea per cup should be used. Black teas should be prepared with boiling water and steeped 3-4 minutes. Black teas that will be served with milk or lemon should be steeped a little longer, 4-5 minutes.
While green tea usually loses its flavor within a year, black tea retains its flavor for several years. For this reason, it has long been an article of trade, and compressed bricks of black tea even served as a form of de facto currency in Mongolia, Tibet and Siberia into the 19th century. It was known since the Tang dynasty that black tea steeped in hot water could also serve as a passable cloth dye for the lower classes that couldn't afford the better quality clothing colors of the time. However, far from being a mark of shame, the "brown star" mark of the dying process was seen as much better than plain cloth and held some importance as a mark of the lower merchant classes through the Ming Dynasty. Traditionally, black tea was the only tea known to Western culture. Although green tea has been gradually increasing in popularity, black tea still accounts for over ninety percent of all tea sold in the West.
A 2001 Boston University study has concluded that short and long-term black tea consumption reverses endothelial vasomotor dysfunction in patients with coronary artery disease. This finding may partly explain the association between tea intake and decreased cardiovascular disease events. In 2006, a German study concluded that the addition of milk prevents vascular protective effects of tea.