The person cant't be Hades because Maleficent and Pete already know about the Blackbox. Also, we can't assume that Darkness is the same person that told Maleficent about the Blackbox. We know he told her that there are other ways to circumvent the law of time traveling, but we don't know if he was the person responsible of telling Maleficent about the Box.
It's possible that it was Luxigbar who told her, and everyone that knows about the Box. I mean, everyone was in a wild goose chase for the Box, yet we see Luxu carrying it with the Foretellers. Whatever Luxu was trying to accomplish by sending everyone in a wild goose chase for the Box, he seemingly accomplished it and got the Box anyways.
...or, y'know, it was Nomura that sent everyone in a wild goose chase for the Box in an effort to give the Box some sort of narrative importance, only for it to be underwhelmingly revealed in the end of the game.