Where does it say in the Bible it's wrong to blackmail?
Nowhere in the Bible does it say that blackmail is wrong. Blackmail is a threat to publicise an embarrassing secret (or an offer to remain silent about this information) along with a demand or a request for funds or other valuable considerations. Blackmail is therefore a threat to commit the sin of gossip unless paid off not to do so.Threatening to gossip (or offering to pay to stop the gossiping) is not the same as gossiping, though. The blackmailer is offering or being enjoined to not commit the sin. In fact, the blackmailee is in a far better position with a blackmailer willing to sell his silence for a fee than in the hands of a gossiper, who will "let the cat out of the bag" no matter what. From the point of view of the backmailee, at least the blackmailer has the decency to allow him to buy his way out of being exposed; the gossiper offers no such consolation1.1 Walter Block: Defending the Undefendable, p. 53 (Fox and Wilkes, 1985).