spoilers:end of spoilers
Given how Blade talks in the Dr. Strange issue about how this event was planned for thousands of years and having all this ancient lost knowledge and magic suddenly at his disposal the story sets up pretty well that he's being possessed. My guess is that by tapping more into his vampiric dark side, it's made Blade more susceptible to outside influences; especially those related to the Darkhold. And given Adana's fixation on watching Blade tap more into his dark side I'm guessing she might have had a hand in this as well. Like she knew this was going to happen all along or something.