I believe philosophically they try to avoid too much of 'use this but take x2 more damage' as they removed that from many spells already, or like how our tank stances are now baked in and you don't choose to either take more damage or deal more damage. I think for making the game approachable this is nice, for those who are more skilled I feel like some sort of punishment choices can be interesting but conflicts with approach-ability, baked into the base job making it either a subtle mechanic or making the job not as approach-able. I mean they could always go back on that, but it would make the job more advanced to play and less casual friendly I think.

I don't think there is anything that stops using drain like spell concepts though, except either it not making something interesting to play (if they try it and it isn't fun- this is probably most important, as they've said before gameplay is first priority). Making it work through lore or whatever only takes creativity, which they don't lack- besides very rare specific cases lore is hardly ever the problem (like we've seen with summoner, beast tribes, and other points- they're able to adapt to what they want to do). So you could have a 'blood mage' but I don't think you'd get using your hp to heal things, at least not likely- if they want to do it then cool I'm not to say no lol. Also to note you'd have some other concerns about hp being a resource in that jobs that could heal the healer could extend the healer's resource pool in a way other healers wouldn't have access to which may make weird relationships with things like tank heals or SMN's regen. Would SMN + tank cure = never worry about my resources again? Might be a solution to that somewhere, but just wanted to bring it up as a potential concern.

Mechanically I think if you've focused on drain spells that it would work better as a tank but we've already got Dark Knight who is partial black mage, at least stereotypical in ff sense, so you could just give them more drain skills that they already have lol. (heal on hit). Thematically however you could easily have blood meters and such, using x spells gain blood, using y drains blood- which could work for any role. Drain spells could even be part of that job's skill set, just think it might be easier to balance if it the resources were still mp and not hp. Heck what you might do that makes it similar yet still balanced to mp is have the healer's attack spells drain hp and mp, such that the healer's resources DO come from draining but only from their own move set (rather than from their hp) and their mp management would come from properly casting certain spells onto enemies (some being dots and instant ogcd for reasonable management in intense situations).

With all that's going on I feel like Chemist seems like a very likely next healer but I think a dark theme'd healer would be cool. Back when people were more on how the healers have simple attack options I had suggested a healer that had some more offensive options, and was dark theme'd, just as a sort of fun goof off moment (Dark Magic Theme'd Healer - technically I mention blood magic in that thread too lol.. well and necromancer but due to the huge hurdle that would be lore I'd never suggest raising the dead, more thinking on the lines of soul magic, curses, poison, or what have you).