If a Blu-ray has been unwrapped, but was sealed when bought, then you can't return it, because they won't be able to sell it again. If it was a more Blu-ray/DVD centred store like HMV then they might be more flexable, but for a store which doesn't deal in them as their primary stock, I don't think they'd accept a return.
The only thing I can sugest is to say it's faulty, as I've done that hundereds of times and it works, although I always feel bad about afterwards.
