A US intelligence agency nowadays would just issue a National Security Letter and force you to not say anything otherwise you go to jail. Most people really don't want to go to jail. An intelligence agency would really only resort to blackmail or extortion if they were operating in a foreign country where they couldn't outright bribe someone.
I imagine that is defeated by simply saying "I refuse to lie", which could be career suicide but it's still an option. I can see how legally they could say "you can't reveal that information, it's a matter of national security" but I don't quite think we're at the point in America where they can go "Say this or go to prison".
