Blackbeard's body itself is darkness. He is a Darkness Man. Imu would be presumed a Something Else Man depending on what his Devil Fruit is (if he has one)
I think I'm imposing a position on this. Sorry about that. But since I am, then I may as well elaborate on it.
This creates a degree issue. Blackbeard represents the concept of a Darkness Man, but Imu can represent something like a Satan-Satan Fruit which could mean he commands darkness or something. This complicates the matter.
For now, my position is that Blackbeard has the purest sense of being associated with darkness. He even says the Devil Fruit chose him. That means it's like fate for him to be whatever the Darkness wanted him to be. That's very convincing to me concerning the degree of his association with darkness.
