Black diamonds

One can find the information about fancy colored diamonds: red, green, blue, yellow, brown, pink. In some instances different minerals involved, in others heat and radiation, or all of the above. I hear people talking about black diamonds. However, I could not find any information about black diamonds.

What is a black diamond?
What makes them black? Are they so heavily included, so they look black? Or something else?

I trully appreciate your efforts.
Regards,
Tanya.