It’s generally not blackmail to threaten someone with some action that, by itself, is legal. See, for example, the many civil settlements where rich people stop the people they raped from going to the police. And I’m unsure what the status is on publishing salacious private photos. Sure, there is a copyright issue. But that’s a civil matter, not criminal.

If it were a crime, it really wouldn’t be much of a threat because they’d be unlikely to follow through, would they?

Edit: there is one federal law where threatening embarrassment is enough, but that requires a demand for money or property, which this is explicitly not.

And because the downvotes are already rolling in: Note that I find this just as abhorrent as you do, and that sexploitation should absolutely be a crime. But the legal situation unfortunately is not at all as clear as the moral one. And the replies below are mostly embarrassing in that nobody even seems to have a clue on the difference between civil and criminal law. Carry on then...