I'm really interested in how both Magic and Combat Arts work in 3H; I find it really interesting how they seem to be counterparts to one another; with each unit seeming to have a learnset when they level up their levels of the specific weapon type; unique to each unit. For example; upon reaching C+ in Axes; Edelgard learns the Monster Breaker Combat Art while Caspar learns the Wild Abandon Combat Art. It will most likely be the exact same levelling both Reason and Faith Magic.
As for the "split" in Reason Magic; I'm pretty sure that both types of Reason magic(Black and Dark) can be learned by levelling the Reason rank; but the type of magic that's focused on in their learnset depends on the unit; for example it seems like Hubert will lean more towards the Dark side of Reason magic while Dorothea looks to be leaning towards the Black side of Reason magic. I'm sure both types will be usable on a unit when the class is able to use Reason magic; it won't be an Awakening/Fates situation where specific dark magic spells are locked on mages that aren't dark mages; but Dark Mages are just mages that just sway to the dark side of Reason, in terms of the class skills granting access to specific dark magic spells.
EDIT: Screw it; let's talk about skills in here too; since they're sorta linked to the Magic system I guess.