Good questions all. And as far as blackmail in this day and age of no morals to speak of, what is blackmailable (if that is a word) must be pretty bad.
What’s blackmailable is: “We don’t have to tell the IRS about your secret bank account if the price is right. Please send whatever your conscience believes is necessary to hclinton@slushfundserver.net. A good starting point for your conscience is the biggest number you can think of, and put two more zeroes on it.”