Um, well...

There's no reason at all why darkness as an affinity would hurt the mage with it or really have any negative stigma at all. ^^; Affinities are literally just what you're best at. Darkness as an affinity would be things like... well, shadows, the absence of light. You'd be good at spells involving that kind of thing. To be hurt by your magic you'd have to actually learn spells that would do that, for whatever reason.

I don't even think Darkness would be that uncommon as an affinity. It's certainly not a rarity like Ether or a totally unique affinity like Death or Mana.