Blood mana is only in act 5, from either the succubi or Baal. If you have more life than mana, they'll instead cast another unique curse which reduces your defense. The oblivion knights don't care at all how much mana you have.

I'd note, also, that, while you'll only be targetted with blood mana from those act 5 monsters if you have more mana than life, you'll still be affected by the lower-defense curse if they cast it on your merc.