Isla channels all the energy from Dean, she channels his death. There is no blood, which is unfortunate. It would have been better if he was taken out in Axel’s rage and not so quickly and silently, so cleanly by her own hand. As it would provide her with a stronger source to draw her magic from. But death is death. It is all a source of dark magic, she needs it as well as his alpha life force, because there is one thing a wolf can do better than a witch, no matter how much she hates it. A wolf can communicate better with the rest of the nasty mutts than she will ever be able to.
There is enough blood around from the rogues in Phoenix's wake to channel the extra power, but taking it from so many weaker sources as well as Dean is more time-consuming and will never give a coherent power.
