I think the myth/mystery is too far into the past to truly discern an answer. But as you've so elegantly put, our life source, our blood that courses through our bodies is too strong of a source for fiction not to be born.
Take a look at the bibles obsession with blood. Blood on the altar, in cups, on the ground, etc. It is ingrained in the very fabric of our molding from the beginning.
I think the vampire monks/priests idea that you and James have come up with is absolutely brilliant. I cannot wait for Innocent Blood in December. What a great Christmas gift! Thanks guys! ;)