Instructions
Enter the Dark Cave from Route 45. The easiest way to get to this path is to have a Pokemon use the Fly move. Fly to Blackthorn City and head south through Route 45 to the mouth of Dark Cave. The opening will be on the right side of Route 45.
Navigate the cave and head to its northwest corner. Be prepared for the wild Pokemon you will encounter along the way, such as Wobbuffet, Golbat and Graveler. You will also need to bring along a Pokemon that knows the move Surf, which will allow you to travel over the pools of water inside the cave.
Talk to the man who is wearing a pair of round shades in the northwest corner of the Dark Cave. He will give you a set of BlackGlasses for your Pokemon.