The Black Refractor is almost certainly referring to Necrozma.
Refractor means "a lens or object that causes refraction. Refraction is when light is deflected by travelling through a substance. Prisms are commonly used to refract light. Necrosma is classified as the Prism Pokémon, knows a move called Prismatic Laser and has two bigass prisms on its arms. On top of that, Necrozma is black and is found in Ten Carat Hill on Melemele Island, matching Looker's description to a T. It is also theorized to be from the same world as the Ultra Beasts, and is only obtainable after completing the Ultra Beast missions. I think it's fair to say that Necrozma is the "black refractor" that Looker describes.