If you're talking about a black Labrador Retriever, then there isn't really a special name for them. Some people might call them a charcoal Lab or a shadow Lab, but those are just nicknames. Officially, they're still just simply black Labs.
The same goes for other colors of Labs too - there aren't any official names for the different colors beyond their base coat color. So if you've got a chocolate Lab, he's just a chocolate Lab; if you've got a yellow Lab, she's just a yellow Lab.
But that doesn't mean that people haven't come up with creative names for all sorts of different colored Labs! For example, some people might refer to their blackLab as an oil slick because of how shiny and sleek their coat is. Other common nicknames include Raven or coal.
No matter what you call them though, one thing is for sure: Black Labs are absolutely beautiful dogs!