Black English

Black English, I have a great interest in knowing how it came from. I heard that it's kind of informal English, and it seems that many teachers don't want students learn it or even use it. Why?

Is it just because Black English is informal and dirty English? Or it's because the Black usage is different from modern English? Or it's about color? I wonder how Americans (not Black) treat Black English since I think there are a lot of people there, not only Black, use this kind of English. These are the questions that I really want to know.