For example, those being blackmailed might not know or have any proof that it's FBI who's blackmailing them. The FBI of course has endless opportunities of plausible deniability, starting with "rogue contractor".
It all boils down to incentives: what exactly do you have to win from disclosing? What is the chance that people will believe you given that you have no evidence? And what would you even expect your audience to do about it? And how does that scenario compare to the worst that your blackmailer is willing to do?
It would be fairly easy for someone with the power of the FBI to rig the subject's incentives heavily in their favor, and to only pursue blackmail on subjects where they are fairly certain of success. At some point of proficiency a casual observer might not be able to distinguish it from lobbying!
We don't know if these things happen or not, but they're entirely possible. Stranger things have been leaked and declassified over the years.