Darkrai is impossible to catch in any main Pokemon games without hacking or glitches, and this includes black 2. However Darkrai is available through events, and you can also obtainable in trades. Here's more info on darkrai if you want it. http://bulbapedia.bulbagarden.net/wiki/Darkrai
NO. Darkrai is an event exclusive Pokemon, and there are no current events for it.
