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.
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:-)
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.
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.
By trading for it.
impossible