Black Hero/Black Person = Cool. Dark Hero/Black Person = Cool. Dark Hero/N = Cool. Dark Hero/Keldeo = NO!
It's too early for him to show up in the Anime. Then again, this series is "fast-paced".
However, I can see Keldeo with his own movie including the Musketeer Trio, sort of liek that Mickey Mouse one. So this "Dark Hero" is not him.