That depends on the jurisdiction. In Canada, for example, it can be libelous or defamatory to publish embarrassing secrets about someone even if they are true. That is, truth is not an absolute defence to libel.
I don’t think this is correct. The victim’s actions don’t make out the elements of the crime. The blackmailer has to demand something.
double post
I read in the times, the legal way to blackmail someone. I guess it cannot be just anyone, though, someone with whom you have had dealings. You institute a lawsuit against them in such a way that the embarrassing (or illegal behavior) will inevitably come out during any deposition or trial. Their lawyer contacts your lawyer and offers to settle the suit and you accept the “settlement”.