In actuality, Dark Paladins have been done before, and they'd proably translate more into a Dark Knight in World of Warcraft. Instead of using diseases and blood magic, it'd be straight up shadow magic. Maybe like a Shadow Priest in armor, instead of an Affliction Warlock in armor. It's been done before, and it wouldn't be terrible on its own. The problem is, we already have Death Knights. Shadow Priests and Affliction Warlocks are already terrible similar, mechanically. We don't need it to happen with Death Knights and Dark Paladins.
Of course, this whole post has been written in the perspective as the Dark Paladin as an entirely separate class.
In World of Warcraft, Dark Paladins don't make sine, because a Paladin can use the Light for evil-doing just the same as a righteous Paladin. As someone else said, it's more of a lawful thing, rather than a good thing.