My first thought was that "black diamonds" was the universal slang for coal in the anthracite regions of eastern Pennsylvania, back when there was a lot of money to be made from the coal trade. But it's also worth noting that the artificial diamonds used for industrial cutting tools and drilling bits are black and opaque -- no point maing them look fancy when they're just used to grind things up. They're every bit as hard as gemstone diamonds (the hardest substance on the planet), but not especially attractive to look at.

As for black gem diamonds, I'll defer on that to the others who have already posted. I never knew such things existed.