So in 5e there exists the "Oathbreaker" paladin subclass precisely for paladins who have fallen out of favor, much like the blackguard class of prior editions. The Oathbreaker subclass specifically gives powers over fiends and the undead. In the goblin fortress there is a book that talks about Oathbreakers so at the very least the game acknowledges that they exist, but whether or not they will be playable is up in the air.
If you just want to be a "dark paladin" I'd suggest rolling up Vengeance Paladin when the paladin class launches- that's almost guaranteed to make it into the game. I think/hope that an Oathbreaker subclass may be modded into the game if it doesn't make it in, but at the very least the game acknowledges that they exist.
