I think the flag of the Blackbeard pirates is a clue to how Blackbeard has more than one devil fruit. I dont know how he got the power from the dead but I think I know why it worked. The flag represents the 3 types of devil fruit. I think he awoke Yami Yami no Mi Logia type then "ate" the Gura Gura no Mi Paramecia type. I think he will go after a Mythical Zoan type like Marco after he awakens the Gura Gura no Mi.