It hails from the RPG element from Warcraft, where there were many more (prestige) classes and titles. You could say it's officially non-canon, since the Holy Light and it's usage developed in a completely different matter since The Burning Crusade.
Dark Paladins are Paladins that not only have strayed from the path that is required to uphold the use of their powers, but have made a pact one way or another with dark powers that directly oppose the path of Paladins. Sometimes this happens when they've done an evil act, and find them self on the long quest of redemption, and fail at it, to which they progress further away from redemption that they eventually seek out evil sources. Sometimes, it's simply because they are raised this way, having no other knowledge of the path to Paladinhood other than by evil means, where judgment and retribution hold key over peace and good.
In World of Warcraft however, we can assume that almost all Paladins are, in one way or another, Dark Paladins. No, not because they are evil, but because the story of the Holy Light simply took a completely different turn. It went from a religion and a philosophy to uphold the use of such good powers, to a source of magic. It can be used by even the most vile person, as long as they know how to "tame" it, and people who are self-righteous are still eligible to use it.
The only race that uphold the virtues, and the only class that uphold the virtues, are Draenei and Priests respectively. All other Paladins have one way or another, used the Holy Light and Paladinhood to smite, judge, repent, and all those bad words you can use when you talk about the Light, against foes they think evil, instead of letting the Holy Light decide for them.
I'd really like to see the Three Virtues canonized again; Paladins are somewhat of a heretical belief towards what the Three Virtues actually stands for, and so far only Uther has been most compassionate in upholding not only the will of the Holy Light, but also the Three Virtues of which Paladinhood originates from.