Certainly blackmail is subjected to an unwilling or at least reluctant victim. If the victim gave whatever you want willingly you obviously wouldn't need blackmail!

The problem I was hinting at (that I stumbled across as an author) is this: How do you push the victims boundaries in a way that it remains believable that she will fullfill the blackmailers demands instead of turning him in.

Is it only neccesary that the 'dark secret' she is hiding is vile enough to accept anything? That is very doubful.

Many authors like to put their victims through fates "worse than death", so to speak and ruin the suspension of disbelief that way. Thinking in the position of the victim, the reader realizes that the story loses it's touch with reality at that point.
I found that a 'reluctant' victim is much easier to put through such suffering; She is actually liking what is happening and thus rationalizes that she has no other choice because she is blackmailed instead of admitting her own hidden desires.

That takes the 'nonconsentual' feeling out of the story however, which is a huge turn off for some people (not me, I'm fine with 'reluctant' victims).

So how do you go about putting your unwilling victim through anything you want without her turning you in?

A cent for your thoughts.

Satan_Klaus