Dark Glasses in Pokémon Sword and Shield will give a boost to any dark type moves the Pokémon has. On the Isle of Armor in particular, this can be useful for your Kubfu if he has evolved into Urshifu through the Tower of Darkness. Since Urshifu acquired by this tower are fighting and dark types, this is a useful place for glasses. However, they will be useful with any Pokemon that has dark type moves. Here's where you can find them on the Isle of Armor.
Related: All Alolan Diglett locations in Pokemon Sword and Shield's Isle of Armor
How to get dark glasses on Isle of Armor
The pair of dark glasses will be placed on the ground in the wilderness area. From the Calming Wetlands, the area where the Mustard Dojo is located, head east to the Brawlers Cave. There will be a lot of items and Pokemon to grab here, but for the glasses you will have to go up all the slopes in the middle of the cave until you come out onto Challenge Road. At the top of the slopes and staircase is the previously mentioned Tower of Darkness. Just walk in the back and there will be two items on the ground (as well as a Dynamax raid lair). The red on the left is the black you are looking for, while the yellow on the right will be a TM.
Now that you have the glasses, make sure you give them to a Pokémon that is experiencing at least one dark move, otherwise it won't do anything for you. Just a sample of dark type moves include Payback, Bite, Crunch, and Sucker Punch. Take advantage of your power and be sure to keep checking back with Gamepur for more Pokemon guides.