The blocks you see is called "tiling". This can be caused by a number of reasons, but often it's related to a weak signal. This could be caused by something as simple as a loose cable connector, or splitting the signal too many time causing signal loss. The installer should have performed a signal strength test at every end point to ensure that video/audio would not be compromised due to weak signal strength. I would try cable connector tightening first, then unplugging the box for 20 seconds to re-boot the box to see if this clears the problem. If not, and the tiling is such that it's on many channels and occurs often, call for a field tech to check it out.
You will find that having Verizon FIOS is a double edged sword. While providing what's regarded as the best video quality available, with alot of bandwidth due usage of fiber for delivery of alot of HD content, their network is a bit fragile requiring resetting of the box by users more often than is desirable. Also their customer service stinks, frought with cases of wrong information and broken promises. Read some of the postings on this site or the DSL sight and you'll get the idea. So far I'm so happy with the video quality and content that I'm willing to puit up with service annoyances.