The best blackmail scenarios have nothing to do with sex; they have to do with something
provable and criminal that the blackmail victim has done, preferably something that will see her sent away for a long, long time.
Once the first blackmail succeeds ("fuck me or this goes to the cops"), then the blackmailer can slowly, carefully force his victim into more and more depraved activity until, finally, she'll do anything for him, either because he's got evidence of her doing more and more illegal and embarrassing things, or just because that's what she's gotten used to.
But you've got to start out either with something reprehensible that your victim has
actually done, or something that can be convincingly faked and that will so terrify her with threats of prison that she'll take that first step and fuck the blackmailer "just this once, to get rid of him."
Then the
REAL fun can begin.
