Eugene Volokh presents a paradox about blackmail in response to a letter someone sent a senator who was planning to vote for Alito that threatened to reveal that the senator was gay unless he voted no, a pretty despicable act (whether the senator is gay or not). The paradox is as follows.
1. Free speech rights allow me to publish embarassing information about someone (in many cases).
2. There's nothing immoral or illegal about asking for money in exchange for a service (in most cases).
3. But when 1 and 2 are combined, we call it blackmail and make it illegal. How can it be that the combination of two legal acts could make something illegal?
As I said in the comments on Eugene's post, there is a moral issue that comes in once you combine the two issues. That issue is what we call coercion. It's not coercion to make an offer to do something positive for someone if they do something for you. If they turn you down then you are no worse off. If it's wrong, it would have to be on other grounds. But if someone threatens you with a negative consequence if you don't do something for them, you are indeed worse off if you turn them down. That undermines the consent of your doing the action and thus puts it in a category with coercion. It's not coercion in the sense of being forced to do something with absolutely no choice, but it's like being forced to choose between a negative consequence and doing the unawanted action. That's indeed what happens when someone puts a gun to your head, so it's coercion in that exact sense. You can risk taking the bullet and not do what they ask, but it's a huge risk. The greater the risk, the greater the coercion.
As a non-lawyer, I can't comment on the legal issues, but that's the moral issue that makes combining 1 and 2 immoral while 1 alone or 2 alone is at least less immoral or even not immoral (depending on the circumstances, perhaps). These are the sorts of moral issues that laws often rely on. So I don't know if it's really counts as a paradox, or at least if it does then it's one that's easily solved.