I can believe there are shadow organizations that lack oversight. In fact, it seems likely.

But that these shadow organizations are maintaining power by surveiling and blackmailing congress people is a whole other ballgame and seems highly unlikely to me.

For one, it's likely that not every Congressperson has some deep dark secret that makes them blackmail-able. And pissing off the only group of people that could cut your funding and expose your shadow organization as well as bring oversight is not the group that you want to be pissing off.

I think game theory would indicate that your only chance of maintaining a shadow organization in the US gov for any length of time is going to be secrecy and maybe some heavily funded lobbying.