When I say black mail, simply knowing that one is routinely snooped on makes people more compliant. The implication can be enough. Had a few joints at college? Want to be a politician? Well, behave and be nice to security issues then, just in case... Very much the Soviet trick. Have every one assume they are spied on.
Not a US problem as such, one for the any relationship between security and politicians (and other powerful people), regardless of country or political system. And all because security insists on secrecy, which I am not convinced is required in a vast majority of cases.
To my mind, the lack of any real reaction to the NSA revelations, both in the US and here in the UK, increase my suspicion that this implied blackmail is rife, perhaps standard practice.