The Darkslide is a trick invented by Rodney Mullen. The first person to do a Darkslide was Mark Gonzales he did a caveman to darkslide in Blind Video Days, Rodney Mullen was the first one to do it using only his feet.
It is an extremely difficult trick in which you grind the board upside-down so that it is on the grip tape.
The Darkslide can be executed in 2 different ways:
Skate towards the rail normally as if you are about to pull off a regular boardslide.
Ollie at the rail, but use your hand and grab the board upside down and slide it upside down on your grip tape.
Land the trick by flipping the board the right way up.