The showrunner is Scott Buck, showrunner of Iron Fist, and one of the minor points of controversy is that Danny won't ever wear his mask, because like Logan's director, Scott Buck thought that it was too silly and 'couldn't think of a justifiable reason to give him it' (which is frankly, a little silly given Danny would be trying to hide his vigilantism), so that makes it look like he's reluctant to give them their masks.
And, you know, Black Bolt wears a mask. In fact, he's one of the few Inhumans I can think of who do so regularly, and if he couldn't think of a reason to justify a man engaging in vigilantism while maintaining a secret identity would wear a mask, I'm guessing he probably couldn't justify a mask for a king of a secret hidden kingdom of superhumans.
There is, of course, plenty of ways to justify Black Bolt wearing his mask, such as:
- Royal headress; being a fictional culture it could be explained as the norm of their monarchs to wear such, similar to a crown of sorts. Expanding that...
- Just straight-up fashion; really, any odd fashion items the Inhumans wear can be explained this way; as a fictional culture, one could claim that its just part of their normal fashion to wear masks, and Black Bolt happens to like the look of it.
- Power direction; Black Bolt's 'tuning fork' on his forehead is generally used to direct his energy powers so his mask could be used as a means to control his power. Similarly...
- Power dampening; Black Bolt's power is extremely volatile and he could reasonably engage the likes of Hulk in battle. The mask could be a means to weaken his power to a more controllable level.
- Armour; as Black Bolt is a king who leads from the front lines, a its very possible to cast his look as if its royal armour. Give it a more protective, armoured look, and it could be turned into a pseudo-knight's helmet-like item, to protect him in battle.
- Silence; Black Bolt can not talk, not because he's incapable of, but because of what happens when he does. Namely, shit gets fucked. If his mask is modified to resemble his Ultimate version where it covers his entire face, it could be a means to keep him silent and stop him from accidentally speaking.
- Protection from Maximus; Black Bolt's brother Maximus is a psychic who likes to manipulate people and has desires for the throne himself. As he's a pretty big menace to be weary of, and the potential dangers of a madman turning Black Bolt into a weapon, it'd make sense if Black Bolt wears a helmet designed to keep people out of his head, Magneto style. Inversely...
- Manipulation from Maximus; the show is having Maximus apparently act as a loyal, loving brother who just has desires for the throne himself. Given this, its entirely possible Maximus could manipulate his brother into wearing the mask for any of these reasons, but the truth being that its actually something to aid Maximus; perhaps it absorbs the energies Black Bolt creates to weaponise it, or possibly weakens his mental state so as to allow him to be manipulated easier. Hell, it could just be to hide his face so as to be easier for Maximus to steal his place.
- Face concealing; Black Bolt underwent Terrigenisis far earlier than he was supposed to, and the process can often have unknown physical effects. Attlian is also known for being a very bigoted place where physically mutated Inhumans are often mistreated by others (see how they treat Lockjaw as a pet, when he (might) have once been a man). In this regard, its possible they'd want to hide the fact the king has some kind of physical change so as to ensure he remains respected by the people.
- Just give him it; ultimately, it doesn't really matter what reasons you need. I don't think people will really, actually, care that some people in a show about a hidden city of super-powered people who mutate themselves by huffing gas wear masks. Its really the least weird thing about the Inhumans.
Really, the mask is the easiest thing to justify. The hard part, really, would be getting people to actually route for Black Bolt and the Inhuman royals, given some of their more morally abhorrent behaviours (such as, you know, keeping slaves, or their treating Lockjaw as a pet when he was once a human (possibly)).
Basically, don't overthink it Scott; plenty of excuses to give him his mask, and I think for the most part, people would like to see it. I know his actor at least is open to it.