I’m not sure blocking day abilities is something BD needs. I mean… it could lock down a few nasty abilities, but do we want that? Is it necessary? The point here is to give evils an excuse for not using their abilities, not to add another tool to BD’s toolkit.
Also, Butlers could use it to prove, even if it’s silent (wasting an ability is worth it, and they could use it on the king to block Royal Finger, which the king doesn’t need most days.)
For that excuse to work, it also has to be widely-available. If the only evil class that has it is Servants, claims that it was used on you wouldn’t be credible. Giving it to eg. the Scorned instead makes it far more believable (and I’d even give it to a few other people, like the Cult Leader or something, so there’s a decent chance of it flying around and people have to take that seriously - someone who would have an incentive to try and block any BD confirm attempts, so either CL, Assassin, Mastermind, or an NK. Maybe Mastermind and one of the Cult converted classes, since they appear more often than individual Unseen converted classes.)
The problem with letting it target kings, I guess, is that it makes it a much more powerful ability, since it entirely eliminates the king’s advantage in late-game standoffs and removes the pressure evils have to kill him before then. This makes it harder to give it to a lot of people, which I feel is necessary for it to do its job of making day-ability confirmation unreliable - for it to work, there has to be a reasonable probability that someone is around who will immediately block you when someone demands you prove.
Like, no way we could give it to the MM or CL if it worked on kings, since having it be almost guaranteed that would negate one of the key points of kings. But if it didn’t work on kings, it becomes a fairly minor ability that is only likely to be used when BD demands someone proves themselves, since there are few other BD with really important day abilities, and few enough BD day abilities overall that using it blindly isn’t likely to accomplish much.
(Oh, also, it doesn’t work on the Prince. Obviously.)