My TV goes black for a full second, then image comes back for about half a second, then black for one second, and repeats. Exception is that sometimes when the image is relatively static, such as an informational graphic portion with some but not much animation, the blinking stops and the image works normally.

Model is a Vizio E65-E1 Refurb bought in 2017.

Additional diagnostic results. I disconnected all HDMI inputs, same beahvior using either built-in smart TV functions or Chromecast from YouTube. Static menus from the built-in system are always visible and functional unless over top of other video. Paused video still exhibits the behavior. I have rebooted the device and checked for firmware updates, of which there are none that were not installed when this issue started. No known impacts, no changes in the rest of the A/V setup. As far as we know, it was iterally "I changed nothing and turned it on and it was doing this." (I work in IT so I know how often this is not the case. I have asked the questions of my wife and thought it through pretty thoroughly and I'm confident this is so in this case. But we are all subject to error.)

YouTube video demonstrating the behavior on a Chromecasted YouTube video.

https://youtu.be/OPsG2nC6uR8