Whatever SE have planned for possible future Jobs, I think it's a little too soon to be making guesses as to how the lore could allow such magic - technically RDM as it traditionally is depicted, is just a swordsman who can use White and Black Magic without excelling in either. It should also be noted that in FFXIV both White and Black Magic were outlawed because past practitioners of those schools of magic brought about Umbral Eras through their misuse. I presume RDM would have a similar explanation. But then I guess we'll see.