I didn't connect the dots first time reading the spoiler but now that's confirmed that Haki could be sealed into object, not sure about living things though the existence of Poseidon could just that, that weird aura in Zoro's sword is practically confirmed now to be Oden's Haki.
This sealing type Haki which is the enherited will of predecessors could be the key to forge a Black Blade if mastered right.