The tea is called "black tea" because of the color of the tea leaves after they are processed, not because of the color of the brewed tea. The leaves undergo a process of oxidation which turns them dark brown or black. This is in contrast to green tea, where the leaves are not oxidized and retain their green color.

Not all tea is red. Black tea is mostly black. Peppermint tea is green. In fact, tea can probably be made from just about any plant, but there is guarantee one would like the taste of it.