I bought Narcissa assuming I could use her to unlock dark magic bonuses in the game, but when I cast at those areas, nothing happens, and a message comes up telling me I need to know dark magic. It makes no sense to me that she wouldn't be a dark witch.
Even within the Harry Potter universe, there are reasons to believe that just because someone is bad or not nice, they are not necessarily a dark wizard.
Regardless, in this game, Narcissa is not a dark wizard and cannot be used to unlock any of the dark magic sections of the game. The earliest available dark wizard is Bellatrix, available in Year 7, part 2.