for what reason would the NSA want to blackmail us for
If the government tries to pull some stuff that's illegal/oppressive/bad for the country they can ruin the reputation of any opposition movement's leaders and render any form of opposition or movement null.
Kind of what happened with the Civil Rights Movement and J. Edgar Hoover.
Alternatively, if protest activists protest government abuses (torture, bribery, corruption) they can easily be eliminated with blackmail. There was in fact and entire government program about blackmailing people to destroy their reputations.
This can happen on a local level too. Have a video of police brutality or excessive force? Too bad, they know some really bad stuff about you they can blackmail you with. Even worse, they can upload child research to your computer or cloud storage and book you up on some of the worst charges imaginable! The possibilities are endless!