It's probably temp image retention. Just turn on some full screen content with the contrast up high, like around 85, and just leave it on. It could take days for it to lessen.
Maybe you should turn on the eco setting that turns off the panel after a certain time.
Panny won't allow warranty for this, it's in the owners manual. They say BB has a burn in warranty, you might want to look into that if it's not too late.