Vassals in CK3 are grateful to be blackmailed because you are voting for their candidate (what a coincidence!)

[Sabertooth17]

See below when you blackmail a vassal the -15 opinion modifier is totally negated by them voting for your candidate even if you forced them to vote for that candidate with a hook. If anything it should be another -15 for forcing me to vote for his/her candidate.

 

[Emperor 1997]

See below when you blackmail a vassal the -15 opinion modifier is totally negated by them voting for your candidate even if you forced them to vote for that candidate with a hook. If anything it should be another -15 for forcing me to vote for his/her candidate.

View attachment 935327

Or they should simply make it harder to blackmail someone, there is no reason that an AI character accepts an blackmail attempt for shunned secrets, because they can not be imprisoned, for shunned secrets, they only lose a small opinion amount.