Royal Blood makes it too easy for someone to confirm that they’re not the Mastermind or a starting evil. While it’s possible for a royal to be converted, it’s never possible for a starting evil to be royal, so anyone who steps up in the Royal step to replace a king is instantly and uncontrovertably cleared of any suspicion of being the starting Mastermind or the NK.
To open a loophole in this and avoid clearing BDs of suspicion too easily, I suggest giving Aristocrats and Apostles a passive that lets anyone in their faction step up to become king as a Royal if the Aristocrat / Apostle themselves are Royal. (The Aristocrat is always royal, but for the Apostle, this would mean that only converted Nobles would be able to use this ability.)
Possibly this would also extend to the Sorcerer (who would not know that an Aristocrat / Apostle is opening the door for them until the opportunity presents itself - as a reminder, remember that a Sorcerer who becomes king turns into an evil / cult king, not a psycho king. Giving them the option to step up would reveal to them that an Aristocrat / Apostle exists, but that’s not really a big deal.)
Having the Mastermind step up to become king would be a huge risk, obviously. And this wouldn’t be something that could be done easily (you’d need a converted Noble who hadn’t become Assassin / Cult Leader, at the exact moment when a new king is chosen.) But I think it would add a new range of potential claims for roles that currently have trouble claiming, and would give people another avenue to cast doubt on a newly-elected king.
Possibly, this should extend to a Possessor having Royal Blood while possessing someone who had that feature.