Most likely today such direct blackmail of (domestic) politicians and other officials is not the case but you can't really rule it out completely.
The bad thing is that the reputation of the FBI and other such agencies is now such that an analogy with the Hoover days can no longer be rejected out of hand 'because they'd never do such a thing'.
They did in the past, for many years with many people, we know that for sure, and they quite possibly still do, see the thread linked here, after all what use could a decade old gag order have and do you actually believe that 100's of thousands of gag orders and NSLs can actually have a legitimate foundation?
That's to me beyond belief, that there are 100's of thousands of instances where the authorities issue permanent gag orders with respect to information gathering on individuals and that this is all somehow both legal and proportional use of powers vested in these agencies.