|Dark Cave, near the Route 45 entrance. Received from the man in the northwestern area of the map. Surf is required.
You can get the BlackGlasses from a man wearing sunglasses deep in Dark Cave’s northern section, accessible from Route 45.
He’s in a secluded area to the northwest, past an underground lake.
If you have Pokémon that can use Surf and Flash, you should be able to get this item shortly after reaching Blackthorn City for the first time.
As its name suggests, Dark Cave is pitch black. You’ll need a Pokémon that knows Flash to light up the way – and you’ll need a Pokémon that can use Surf to reach the NPC with the BlackGlasses.
You can get HM05 Flash in Violet City’s Sprout Tower by defeating the head monk, Sage Li. He’s on the highest floor, past several other monk trainers.
Getting the BlackGlasses
Start at the Move Deleter’s house in Blackthorn City and head south onto Route 45.
Stick to the left-side wall to find the entrance to the Dark Cave.
Once inside, have your Pokémon use Flash to light up the cave.
Now that you can see, you’ll find the underground lake to the southeast. Use Surf to enter the water and continue going south.
Stick to the lake’s western edge until you find dry land again. Get out of the water and start heading north from here.
Climb the stairs and turn left, continuing further into the cave.
Go down the next flight of stairs and immediately head northwest. We’re almost there.
You’ll find a guy wearing sunglasses at the end of the road, a little bit to the right. Talk to him to get the BlackGlasses.
The BlackGlasses is a held item that increases the Power of any Dark-type moves in the holder’s arsenal by 10%.
This can make powerful moves like Bite or Crunch even more potent.
The ideal BlackGlasses holder is a top-tier Dark-type Pokémon like Umbreon or Tyranitar.