It isn't debating to answer a question with an unrelated question, Extortion involves the threat of violence, blackmail does not.
If I am extorting you the threat is based on my actions, if I'm blackmailing you the threat is based on your own previous activities.
Not necessarily true. You might find yourself being blackmailed for your parent's or a loved one's or even a business partner's poor judgement.
Additional, the blackmailer is not bound by the usual requirements to uphold their end of the bargain. After making the stated payment, the blackmailer can easily pressure for additional or larger payments. They can also betray and spill the beans anyway, after collecting the cash, by selling the information to gossip publications or elsewhere.