afaik some adapters - like apple‘s - are limited to 30hz. you need a hdmi 2.0 compatible adapter, tv and cable. even then it‘s probably easier to do this via displayport (and adapter, if necessary).
@ refresh rate - there should be no difference in sharpness between 60hz and 30hz, but if your tv interprets the refresh rate (in combination with the resolution) wrong, or one point (cable, tv) is not compatible this could result in a blurry picture.
rec 709 is the color space for hd - if you want 4k hdr, you need to set something different (eg. bt2020). but most youtube videos are in 4:2:0, 8 bit chroma-sampling anyway, rec709 should work fine, i guess.
those blocks are compression artifacts - youtube’s compression reduces the original file by a factor of - i don‘t know 100? - in comparison to an uncompressed 444 stream - so something‘s got to give. remember, one already heavily compressed blu-ray 4k movie has about 66-100gb, whereas a youtube video of the same length has about 6-10 gb. you could lower the black level of your tv, so you don‘t see those artifacts in the dark areas. almost all tvs are factory-set to a much to high brightness-level (also called „torch mode“, to stand out in the shop) anyway, so you‘ll see a different picture than the producer of the movie color-graded for.
[doublepost=1526561510][/doublepost]btw., turn off all the „noise reduction, motion smoothing, contrast optimization, probably even „upscaling“,... crap on your tv. that puts you at the mercy of the software in the tv - e.g. samsung is famous for it‘s crappy sw - and shows you the pictures not as they were intended to show. this could also be the cause of your problems.