How to get rid of these color blocks?

[protoX]

I'm trying to compress a 720p file to a bit more smaller size (with MeGUI v/a x264) . How do I get rid of these color blocks?

[hech54]

I don't see any "color blocks" - even in the large image.

[protoX]

Watch it in full brightness/contrast.

[jagabo]

Set a higher qcomp value. But you won't be able to get both small size and retain all the noise that prevents that type of posterization.