Hello, In order to tell if it's Comcast or the TV itself, we need to isolate the problem from the box- by either:
1) Trying another device such as a DVD to see if the issue's still there. If it only occurs with the Comcast box- then the issue is the Comcast box.
2) Grab the original Sony remote and press HOME/MENU- and let me know if the Sony MENU appears clearly OVER those square and on the screen when it's black. If the MENU is also affected/covered by the black boxes, or black screen prevents the MENU from displaying- then the TV is the issue.
If you've isolated it down to the TV, then you're looking at either a Mainboard, or LCD controller failure, which requires testing and replacement of the offending board. Average cost for this repair is in the $250 range on this set, which- depending on how old the set is, may not be cost effective. However if you'd like to have the set repaired, you can use the link below to locate a tech in your area:
If you need further assistance let me know. Please do not forget to click a POSITIVE RATING before you go- as it's how I am credited for my work. You can follow up at any time, at no additional cost.