Is there a moral difference between blackmail and an NDA?

Reader, may I call you John? Now imagine, John, that you are my employer and I know (or claim) that you made an inappropriate sexual advance towards me in the workplace. So I approach you. ‘John,’ I say, ‘you groped me in the lift. Give me £5,000 or I’ll make this public.’

That is blackmail. It’s blackmail whether or not the allegation is true. You can go straight to the police and I will very likely be charged with a serious criminal offence.

Now imagine a different scene. You, John, aware that I have been saying this about you, approach me first. ‘Matthew,’ you say, ‘I know what you’re saying about me. I deny the allegation of course, but I undertake to give you £5,000 if you do not proceed against me, and undertake never to disclose your allegation. Just sign here.’

You have offered me a non-disclosure agreement (NDA). There is no blackmail here — or not, at least, of the criminal kind.