Leaving some notes here about Blockout.
When running in the intro screen of 'blockout' (where it says 'press 1') the game is doing too much in one frame and it misses VBL interrupt, this is what causes the screen to flash. It can miss it by a wide margin, up to 2 scanlines, so this isn't something a small timing tweak can fix.
The reason it is doing too much is that it runs extra routines for ball reflections and keyboard scanning, and this is just a little too much stuff.
I looked for evidence of this in actual gameplay videos and found this:
https://www.youtube.com/watch?v=DuMK4VhiwIo
You can see around 2:28 in the video that there is a graphical artifact in the top right corner of the screen. So it's definitely also having trouble on console.
The difference is that this only effects a couple scanlines. I'm not sure what mechanism I'm missing that allows this to happen. On the right track though.
EDIT: It also turns on HBL interrupts and expects them to either not happen or happen only once per frame.