In the U.S. at least, it may not even be blackmail. The precise boundaries of blackmail are a bit murky, but usually it's not blackmail if the debt is legitimat

In the U.S. at least, it may not even be blackmail. The precise boundaries of blackmail are a bit murky, but usually it's not blackmail if the debt is legitimate and the threatened retaliation is just to announce that the debt wasn't paid. That is probably true even if the announcement is intended to hinder the debtor's future business. Example: if a company writes you a letter demanding payment of an overdue bill, and threatens to put a black mark on your credit-report if you don't pay by $date, that isn't blackmail. That said, it is probably better to avoid being near murky lines.