If a monster is immune to your element you will always deal 1 damage to it and not miss (kinda like how monsters like snails deal 1 damage to you). Try hitting BF with holy attacks and you'll do 1 damage too
This^. As to why those are immune to Holy, from what I heard its because they don't want AM/Bish to start leeching there, so they made those mobs immune to all elements.