Borealis look in black background on HDTV

After last night's ultra dark GOT episode I decided to try fine tuning my tv settings a bit because I've noticed other shows that are a bit dark.

After some tweaking I have the brightness and contrast at a good place but many of the scenes with something like a black sky in the background will have a layering or borealis looking effect.

Any ideas what this is (or more importantly, what it's called) and how to fix it without making everything too dark once again?