I need it for beating the Elite's 4 Chanldure.
If you have multiple Dark Type moves on Bisharp, go for the Black Glasses.
If you don't and rely on luck, then go for the Scope Lens.
And just to tell you Bisharp can actually sweep Caitlin and the Ghost Type Elite 4 without any item on it.
Scope Lens. The additional Critical Rate will be very useful to have, as it can save you in desperate times.
It boosts your critical power which is very helpful when you are stuck in tight times. Although I would use Black Glasses if you have 2+ dark moves since dark moves are super effective against Ghost types.