I highly doubt that you can get him without cheats since it is in a different Gen. For Pokemon Black and White, there was a Darkrai event. And I think that event has ended. But if you do have BW, you can probably do a Poketransfer. And it has been confirmed that BW2 has a Poketransfer system. Then you can have a Darkrai in BW2!
Hope this helps! :)