Since I guess it could be considered an exploit (though you really can't see or do much from up there but it's still fun) here's how to get on the Helicopter in Blackhawk.
First make sure you load up on weapons - Mainly the snipers and at least one double-barreled
shotgun. Oh, and don't jump too much while you do this.
First you have to go outside of the main playing area of the map. There are three areas that I know of, but I'll only mention the easiest. Go up the two sets of stairs of the building with the dome on top and approximately at the "3:00p.m." position relative to the downed helicopter in the center of the map. There, you should see one weapon (usually a Lightning Gun or the blue Skrith Rifle). Go up to the ledge where the electrical wires are and jump on them. Turn left, walk down the wire and to the other pole. Now, walk half way down the other wire and face the building right in front of you.
Double jump and then shoot a shot gun directly below you to give you an extra boost to jump on the roof. Repeat.
Now run forward and you should be out of the map. Turn right and run all the way down that area being careful that you don't fall off the edge! Go up the hill and wait to see where the Helicopter comes up from behind the map. When it comes back, just jump down on it.
There will be some... turbulence
So just crouch until the helicopter comes to a full stop. And that's it! Have fun.