Was ist der Unterschied zwischen rotem und schwarzem Tee?

They're the same thing — it's a translation quirk. In Chinese, the tea is called 红茶 (hong cha) meaning 'red tea', named for the color of the liquor in the cup. When it was first exported to Europe, Western traders called it 'black tea' because of the dark dried leaves. Confusingly, what Chinese people call 'black tea' (黑茶, hei cha) actually refers to post-fermented teas like pu-erh.