Nope.
you cant i think you can not
You can't get him without events or cheats. For Darkrai, Manaphy and Arceus you need a event to capture him.
Yes, you can obtain Darkrai in Pokémon Black 2 through an event. Darkrai was made available during special distribution events, where players could receive the Pokémon via Wi-Fi or through in-person events. If you missed those events, Darkrai cannot be caught in the game without using cheats or external methods, as it is not available in the wild or through normal gameplay.
You would have to wait for a new mystery gift to get Darkrai, it's the only way without cheats.
http://www.youtube.com/watch?v=YRKPj4tcybE
Darkrai.
In sapphire you cant catch it without events or cheats:-)
To get Darkrai simply go to fullmoon island and go on the right side and use surf(or ghost cheat) when you get there the will be an entrance through the trees go through there and capture darkrai!
Get the key from Mystery Gift and open the door in caveport that is locked.
No
To get event Pokemon without using cheats or events is simply impossible without trading. If trading is not an option you must attend these events or use a cheat device.
By trading for it.