Neither Alliance or Horde where ever pure black and white, both are grey imo. However, Alliance is a slightly lighter shade of grey then Horde. Not so much wrong with that imo.
Looking at wow though, the pure white and pure black character where mostly (I daren't say always ) outside either faction. Old jaina is pretty damn white but was never 100% alliance imo. Thrall was always pretty white but got a complete bleach -after- he left the his Horde leadership position. The big bads where always outside the Horde or the Alliance, either from the start (deathwing) or was kicked out before he went completely black (Arthas). I think that now we finally see a change in that. Finally someone within the playable factions is turning black and we have to find a way to deal with that. The Horde because it threatens to tear apart said Horde. The alliance because it's far to dangerous to have a big bad in charge of a superpower.
One of the two factions had to be the first. It ended up being the Horde. I really hope to see that they continue this trend and that hopefully the Alliance will get their share of black soon. Varian going evil would be lame though. I'm rooting for either the dark dwarves taking over the council or greymane going bananas :P