Other threads both here and on XDA Developers give a huge amount of detail on this issue, with some fixes and workarounds. Seems to be hardware related as some GNotes are better than others, gamma levels also looks to be a culprit.
As mine so far has not been a problem I have not really looked at the issue in detail, I believe setting the display Screen Mode to "Movie" helps as this reduces the gamma. On my White GNote (16Gb Dec 11) blacks are deep and uniform with no banding, black crush or blocking. I only see blocking/black crush when the media of very poor quality, low bit rate, or overly compressed.
I am generally playing back MKV`s with a size of around 2.5Gb with the standard player so far the GNote has played all I have tried and I have specifically tried films that are visually dark by nature and they have all looked stunning on the display. Even a couple of movies from YouTube (Virus -
resurrection Day) which also played flawlessly given the resolution, surprisingly good.
So not all GNotes are not effected to the same extent, once I flipped Screen Mode to "Movie" reducing saturation, there is very little to complain about, given we are discussing a mobile device...