I got a shiny Arceus threw Wonder Trading and it knew Hydro Pump, Earth Power, Judgement, and Blast Burn. I am worried it is hacked because of the move Blast Burn. Is this Arceus hacked?
According to this, Blast Burn Arceus is perfectly legal as long as it's from a Gen VI game. I'm more concerned about Hydro Pump, which Arceus cannot learn. Is it possible you confused it with Hydro Cannon? If so then it is very likely your Arceus is legit.