How is the president Trumps payment to Stormy Daniels to keep quiet not extortion? Are you allowed to pay someone to keep quiet? How is that different that for getting paid from a company to keep quiet?
The difference between a Non-Disclosure Agreement (NDA) and Black Mail (such an ugly word...) is really which party came up with the idea. If Trump approached Stormy and said "Hey, if I pay you, will you promise not to talk about our secret affair to anyone." Than it's NDA. Trump is the party that benefits from the silence and is willing to comply.
If Stormy says "Hey, Mr. Presidential Candidate Trump, I know what you did with me that summer... I won't tell anyone, but I could really use some money." Then this is Black Mail.
Thus far the parties seem to agree it was an NDA, and that Stormy wants to back out and return the money. At this moment, Stormy might have breached contract if she spoke up before she returned the money.
If Stormy's suddenly coming out over this having called the President and playing Darth Vader to his Lando (I am altering the deal, pray I do not alter it further... also write me another check for the same value). Than this is once again Black Mail... and because its out in the public now, possibly Breach of Contract. Also a reason why you should always come clean when met with possible black mail. This final scenario is also hypothetical. Thus far there has been no evidence that this has happened.