Black Lenses are rare drops from Demon Eyes. Their sole use is in the crafting of Sunglasses (two are required). A related item, the Lens, is used to craft Goggles. Black Lenses have a 1% (1:100) drop chance.
Another good way to obtain Black Lenses is to make a farm. Since it'll take alot of time, especially because Demon Eyes only spawn during night time, it would be advised to make an AFK farm. You can do whatever you want and return after a few hours or ever so often to check for a Black Lens. Yrimir has a good video showing you his efficient farm.