To consume someone else's blood is to consume some vital part of his or her life. If you consume enough of their blood, you gain their "vital powers" (and, obviously, they die).
Madness string 1670
Do not ever think they come easily, these words upon words upon words... I do not know them any more than you do before they come. I just put them together and then they have meaning. That's the funny thing about language. It is never easy until it comes together.