I hear sentiment like this occasionally and I genuinely wonder if this is conspiracy theory stuff or if this sort of thing actually happened in the past.

I'm aware of the programs Snowden revealed, Tempora / XKeyscore / Longhaul / the like, plus I've heard J. Edgar Hoover did bad things and lots of CIA meddling internationally was bad. Still, these seem qualitatively different to the explicit blackmail you're referring to.

Do you (or someone else reading this) know of historical examples that demonstrate a pattern of this sort of thing? You can interpret "this sort of thing" as you wish.

That's a lot to ask for on the spot, so if not, I would be interested in what generally makes you approach the situation from this cynical angle, especially given that it's the FBI. In my experience, which is fairly limited but is as a US citizen, most of the time the US government mostly follows the law and doesn't do this sort of thing to citizens.

One of the more notable examples:

https://en.wikipedia.org/wiki/Joseph_Nacchio