Cuba's been for a long time the only country the US won't allow its citizens to visit (head, meet desk); North Korea is, so far as I know, the only country that actively prevents US citizens from visiting, although that may be a misrepresentation of the facts on the ground.
Anyway, http://uncorneredmarket.com/american-travel-iran/ has more information.
The typical experience for Americans who have applied for visas to visit North Korea is a very long wait, with only gaining approval (sometimes after years of applications), only to have the approval revoked sometimes as late as hours before their flights depart. Some people report having had their travel visas revoked three or four times before finally being granted entrance.
"Not blocked" doesn't seem like the right term for whatever that is.
TL;DR: Visa applications take longer (but still usually <30 days) and you must be accompanied by a guide at all times, but other than that you're free to travel around.
It's worth noting that this is considerably looser than North Korea, where you not only have a guide, but can only go to the places that have been agreed in advance and your guides permit you to go to.