In the world of magic, there were the affinities that anyone was able to learn and gain affinity with. Fire, Wind, Earth, Ice, Lightning, Light, and even Dark. Learning such affinities, and having certain cores, would make it harder to learn other affinities, but technically any mage could learn all of these affinities, and that included Dark Magic.
Raze wasn't special in that department; he was just ahead of everyone who had tried to learn that particular magic in the past.
Then there were mages who had unique magic traits, special talents that mages were born with, that only they could do. Along with this, there were also unique affinities—mages either born with or later in life discovering affinities that they could use. Not enough research had been placed into how or why this phenomenon had occurred, leading to mages and society in Alterian itself calling these mages the chosen ones.