It grows on Earth too! And apparently...whatever planet Ryslig is on does as well.

[Somehow, he has yet to actually find that information. It's kind of been bothering him. It's almost as though nothing exists outside of the peninsula...]

But black teas are black because the leaves have been oxidized for a longer time than white or green teas, that's all. It makes the color darker and the flavor stronger once the tea is brewed. I don't really know the specifics when it comes to the oxidization process, I just...know that that's what determines a tea's variety.