Should Blackmail Be Legal?
lesswrong.com
lesswrong.com
It's about power imbalance. In the case of blackmail, it's extortion. Someone forces you to pay money and threatens you with unpleasant consequences.
With an NDA, not the person signing the NDA has more power, the other party has. Yes, they pay you, but they can sue you if you leak the secret.
Which, thinking about it, also likely happens if a blackmailer leaks their secret when you don't pay up... So I think the assymetry is not as big as they say. Also a foolish consistency is the hobgoblin of a small mind.
You can (at least presumably) control whether you will break a contract (NDA or otherwise) in the future. You cannot control something that has happened in the past, and thus you have much less choice over your actions if you're blackmailed.