Not true. It would only be blackmail if he didn't legally owe the money.
Threatening to go to the DA is very appropriate. In fact, it's what you should do so that he knows how serious it is. I would say something like, "If there's not a cashier's check in my hand by x date, I will file criminal charges with the DA."
It would be the polite thing to do, so that he's not blindsided with a criminal record.