Except when one admits it.
The whole idea of pardoning somebody for an unknown offense cannot stand. The blanket pardon cannot be Constitutional.
According to https://www.usconstitution.net/presidential-pardon-limits/ , Congress proposed a law (after Clinton pardoned Marc Rich) that would require the details of Presidential pardons to be made public. That suggests to me that the pardons themselves have to state what crimes are being pardoned, even if the crimes are not publicly disclosed. If blanket pardons are allowed, law enforcement and justice are a sick joke.