I would also be very interested to know the OT of said 'Shiny Darkrai', as while I'm sadly unable to verify whether or not that Pokemon can be legitimately obtained that way (from a random trade that is) after it was re-released in the remakes of Gen 4, it really does sound like a 'too good to be true' moment. I'm not going to outright say it was for sure a hacked Pokemon or not, but if it turns out to be, trading it here on the PC is against the site's rules.
Oh, another question. What kind of PokeBall is it in?