It's not blackmail. You're taking $40k in exchange for an unbounded number of problems. Maybe someday some exec will come up with "Oh, we used C++ at Twitter, so answering that question on StackOverflow is a violation of your NDA, please return the $40k immediately." Now you're on the hook for either $40k of court costs, or writing them a $40k check. (I don't think you get the $15k you paid in taxes, either.)

For a few million dollars for a bounded period of time, sure, disconnect yourself from the Internet for that bounded period of time. For $40k, you're just taking on unnecessary problems that you don't actually have the resources to solve.

(I was offered $0 to sign an NDA after leaving my last job. I did not sign it.)