And they all have some blocks. Since the content matters, are you sure this is not a content issue?
Report it all the same. Ask if it's HDR10.
I have purchased the below TV this week and watching bright scenes is OK (not brilliant), but all dark scenes are really bad. I've tried my Sky + HD box, the internal apps and my PS4 using Netflix and they are all poor. All full HD sources.
I have compared it to my old Samsung HD TV, and the picture on that is far better. Do you think this is just how the TV work's, or do you think it's a problem.
Some examples are here: https://goo.gl/photos/zdqWCSd3RfVgr4Pc8
I've tried tweaking settings and modes and nothing really helps. I'm tempted to report it as a problem.
SAMSUNG - UE40KU6400 Smart 4K Ultra HD HDR 40" LED TV