I have good news and I have bad news. Firstly, Cresselia isn't necessary to catch Darkrai. That was the good news. One of the bad news is that Darkrai is an Event only Pokemon. You cannot catch Darkrai in any game legally. That would have been all well but I didn't put the bad news in plural for nothing. Firstly, there are no Darkrai event happening at the moment, in any Generation. So at the moment you can't get it legally at all. But secondly, there are no events for BW/BW2 anymore. As of XY there have been no events at all for the older games (the same applied when BW replaced DPP/HGSS).
So we have an event only Pokemon, who cannot be obtained because there are no events featuring it and there will never be any events in the games at hand. However, that does not mean all is lost.
As I mentioned earlier one can hack Darkrai into the games. Problem is I don't know how to do that and it isn't exactly safe to do. So that is not ideal either.
That leaves us with trading alone. You could trade with someone who had acquired Darkrai in an earlier event online. Problem is event Pokemon cannot be traded online, and you cannot trade online at all anymore in the Gen 5 games as that service was shut down when XY arrived.
(I'm starting to feel like I'm putting out a fire with a water bucket but the bucket is full of holes and on fire and the water itself is on fire, and I'm on fire and the fire is on fire.)
So that leaves us with local trade. The only way to legally get Darkrai in BW 2 is to trade with someone who have it in person. And that strategy works, despite how impractical it is.