Ford’s pardon of Nixon was never challenged in court, and it was tacitly implied that if it was challenged, Ford would still be president for a few years and would just pardon Nixon for any convictions anyway, so the issue was left alone. In my opinion, a presidential blanket pardon isn’t ‘settled law’ at all, and the elephant in the room has just been ignored for forty years.
I agree with you. I would think that a blanket pardon in regards to all federal statutes applying to a certain action might be permissible. For example, and I don’t recall his exact verbiage, but it Ford pardoned Nixon for all crimes related to Watergate, then that might be legal. I can’t see a president being able to pardon someone for the rest of their lives for all federal crimes in the past, present, and future.