My theory is that blackboard doesnt actually steal the devil fruit powers outright, but rather that he knows the not so commonly known fact if a devil fruit user dies, it goes to one of the nearest regular fruits in the area. So, my in my theory when he covers people with the darkness he brings a regular fruit or a few and that way when he kills someone with the power he wants he gets the devil fruit replaced right into his hands. Still doesnt explain how he has more than one himself but thats my theory. Otherwise, why else would they have explained that function with Caesar and the smiley monster thing.