[Gargoyle]

I am replying to this thread from a completely different meaning of you original question - "To Blood Magic, Or Not To Blood Magic".

My answer will also tied to the "Dom III Wishlist" so I should post it there also.

I'll like to see many more elements of the game to be tied to an "Good and Evil" scale. For example, if you were at the extreme evil end of the scale it would benefit your Death and Blood magic but would hamper your Nature and Air. There could even be specific Good and Evil units, items and spells.

So to answer your question, you would use Blood if your were tending toward evil, but not toward good.