I think the idea of supernatural forces possessing people is a fairly common fantasy trope, especially in regards to the element of the spirit or the way in which the person dies being a part of the finished products abilities.

Blood bending is almost definitely against the rules, it is almost completely unavoidable, far too specific and niche a defence is required to protect yourself from it. 

Divine literally means 'having the nature of a deity' that's WAYYYYYY too general to be specifically powerful against a Wraith, where-as Holy is linked to spiritualism and combating 'evil' and essentially linked to the idea of 'God' in a Christian sense, who is the historical enemy of the Undead (Vampires being the spawn of Satan) and Demons (obvious) therefore I like to make sure my opponent has abilities bestowed upon them by a god of this sort before I accept it as -extra- powerful. If your powers come from a god of light for example then by all accounts they are 'holy' and deserve the bonus, if they use 'holy water' or some such its the same concept.

Honestly, I'm not sure why Holy Magic specifically is so under-used in Arena, I seem to be one of the only people who has recognised half the people you fight are evil spirits or undead or demon-vampire-werepoofs. Judgement and my Priest character and even Isaac the freaking Ghoul has anti-evil weaponry and spells.