I think blackbeard ate the mythical wendigo devil fruit.As the mythology of the wendigo strongly draws insperation from werewolf mythology.I also think that's the reason for why blackbeard feelt such an immense pain when ace burned him alive.As wendigos only weakness would be that of the element of fire.Also i think this would tie in strongly with the elbaf arc because the wendigo is said to be a wolf or deer that grows in proportion to the number of people that it eats.It's also very similar to that of the wolf of fenrir within norse mythology.As the wolf known as fenrir within norse mythology also grows in size in proportion to the number of people that it has consumed.The devil fruits ability would also allow him to eat eight devil fruits.Symbolizing the eight moons of the one piece world.Wendigo's are also said to be evil spirits that can strongly relate to that of demons or devils