[quote=@jasonwolf]
[@AdobeFlash] But blood doesn't provide like anything. It's some iron and water and other generic bits for life. I think having limited blood and almost no ability to make blood makes blood specifically usefu.
[/quote]

I think here is a good point to put in a little magic. We could say the blood of a supernatural being is infused with magic, giving it special traits. And that's why many potions call for blood of this, or that. Magic flows in and through [i]everything[/i]. Wouldn't hurt to use magic to explain away a few problems.