I'm not going to give you a detailed answer, mainly because I'm not going to post anything like this on an open-source web site.
However, your antagonists are going to encounter many problems:
1) Roving security guards with live ammunition
2) Helicopters are secured unless they're actually getting ready to lift off
3) Blackhawks don't have the range to get back to the States, no matter how much fuel you load them up with. They can not cross the oceans in either direction.
4) Unless your antagonists are rotary-wing qualified, they're unlikely to get very far, even if they can get the 'hawk turned on
5) Good chance they'd be shot down rather quickly as soon as the IFF for that particular 'hawk was identified. Unarmed helicopters don't do well against an A-10 firing 30mm rounds through it.
6) Helicopters are finely tuned instruments. Your antagonists would need parts and a mechanic to keep it flying for very long.
7) Most helicopter pilots don't fly more than X number of hours a day before manditory rest. You'd need multiple pilots.
8) Fuel supplies are also guarded.
9) Even if they get out of Iran, as soon as they cross another country's borders, they'll be running into Air Defense systems and likely shot out of the sky by anti-air missiles.
However, with good planning and/or a lot of luck, your antagonists might be able to swipe the helicopter . . . but they're not getting home in one.