You’ll want to give your Pokémon items to hold onto that they can use in battle. Some of these held items are extremely useful in boosting their attack moves. For example, Black Glasses enhance the power of Dark-type moves. Here’s where you need to go to find the Black Glasses in Pokémon Brilliant Diamond and Shining Pearl.
You won’t receive this item immediately, and you don’t have to buy them. Instead, you’ll find them in Celestic Town. You’ll be able to reach this town after you’ve defeated the fourth Gym Leader, Wake. You’ll need to find the Team Galactic grunt in Pastoria City and follow them up to Valor Lakefront. After you make them run away, you’ll encounter Cynthia. She provides you with Secret Medicine to heal the Psyduck standing in the path on Route 210.
Once you give the Psyduck the Secret Medicine, Cynthia will appear again, and she’ll ask you to take an item to her grandmother in Celestic. You’ll have to work your way through Route 210 to make it to the city. When you arrive, go to the west side of town and find The Elder’s House. You’ll see a man standing inside the house with black glasses. Speak with him, and he’ll give you a pair of Black Glasses.