Can Khorne play this card on a region where he has no units, just to roll two dice at the start of battle and kill some units off?

The battle phase (p.15) suggests that if no dice are going to be rolled for a battle then the battle does not take place in that region, does blood frenzy ALWAYS happen, battle or not? What happens if only one player has units in the region? What about if only cultists (with an attack of 0) are involved, would a battle happen then?

Essentially...is Blood Frenzy ALWAYS playable and gives Khorne 2 dice...or is it only playable if there is going to be a battle? And what counts as a battle if this is the case?

I would say yes. Battle is defined on page 15 as a player being able to roll battle dice and having viable targets.

If Khorne plays Blood Frenzy, he just gained two battle dice. If he has targets, either other players' followers or Peasant tokens, then a Battle should take place.