Darkslide - A flip into a slide where the board is upside down and you are sliding on the griptape. The definition of dark is on the skateboard terms page. It is pretty much identical to this:
Anything landing with the board upside down is called dark. Anything that involves the board being upside down. There is a lot of freestyle trick overlap here. Rodney Mullen had a few video parts around 2000 that feature tricks with a half flip into a slide and half flip out. People called them darkslides. Lots of other pros have done them but they have remained a bit of a gimmick or dork trick.
They are cool looking but really damage your skateboard. The slide shreds your grip tape. You are also landing on the board where it is weakest. The concave buckles and doesn't resist cracking or breaking when stood on upside down. So darkslide away but you are risking breaking your board.
You need a lot of wax to make this trick slide. Either that or do it of a down sloping ledge or rail. It will take a bit of speed to slide through your grip and on the outer edges on the top of your concave. I would recommend just going for a full flip into a trick. It won't damage you board and it will slide a lot better.
Darkslides; a cool trick but I don't usually bother with them.