Let's look at what both do.
BlackGlasses: (You called them Dark Sunglasses)
20% Increase in power on all Dark Type Moves.
Can be used on all dark moves a Pokemon has.
30% Increase in power on 1 Dark Type Move. (50% in Black, White, Black 2 and White 2)
Can only be used once.
From the information above, I would recommend BlackGlasses over Dark Gem, as you will see more use out of the BlackGlasses.
BlackGlasses | Dark Gem