Black tea is actually a blend of two different varieties of camellia sinensis, camellia sinensis sinensis and camellia sinensis assamica. The first yields shorter leaves and used mainly in China and other East Asian countries while the second has a larger leaf and is used in India & Sri Lanka. When the leaves dry, they become oxidized, turning them black ... hence the name, Black Tea.
Because of the oxidation process, the caffeine levels present in the tea lessen. Comparing different tea, Chinese black teas have about the same caffeine content as green teas where the larger leafed Indian black teas have a slightly higher caffeine content. Both are about 1/2 the content of a cup of coffee. That being said, the longer a tea is steaped, the higher the caffeine content will be. When resteaping tea, you will be lessening the caffeine content.