Logic behind dark coloured BBs is that a sniper should have a perfectly zeroed scope, for the weapon and ammo choice, which should be set against targets (can be any colour, so white paper would allow you to see where dark coloured BBs are hitting) before the game day. Then, when you use dark BBs and are hidden away in some cover, in a ghillie suit, with your quiet rifle, the enemy can't see you, hear your, or see where the shots are coming from.
Problem with that is, it doesn't really work. If you zero your scope one day, then go to a skirmish a few days later, the wind has most likely changed. This makes your zeroing off, so you can't see where your shots are landing, and therefore you can't adjust to make the next shot hit. Some times, when using dark BBs, I'm not even certain anything is coming out the barrel, so I shoot the ground or a tree in front of me, so I can definitely see/hear the impact.
The idea's there, it just doesn't really work in airsoft. If the wind had less/no effect on BB flight, it'd work pretty well.