What's the difference between negotiation and blackmail?

In one of the Bill Cosby cases (about which there is another thread), Cosby's lawyers are claiming that an accuser attempted to "blackmail" him by demanding money and threatening to sue. When does a legitimate claim become blackmail? If you damage my car, and I say "You pay for it or I'll call the cops," is that blackmail? If I say, "You injured me and if I don't receive satisfactory compensation I'll sue you," is that blackmail? I understand that threatening exposure ("Look at these pictures....") is blackmail, but is it really blackmail to advise someone that you will exercise your legal rights if a settlement can't be reached?