My Roku 3 will blink away whatever is the last thing drawn for a moment, but it still shows up in the screenshot, so it's not a problem.
4200X, 7.6.0 build 4125-04
Roku Stick blinking out much more than Roku 3, but again it's still showing up in the shot.
3600X, 7.6.0 build 4125-24
I got no blink of any kind on Roku 4, working flawlessly
4400X, 7.6.0 build 4120-17
On my Roku 1 I got very terrible results! Whole bar sections of graphics are missing, sliced out with reckless abandon, yuck! The game was almost going in slow motion while the shot was processing, and the number of black bar sections seems to coincide with the number of pauses I could notice. I then switched the screenshot format to png, and it got even worse. There were more pauses because the processing time was longer, and there were correspondingly more black bar areas.
2710X, 7.6.0 build 4120-06

My Roku Express+ working flawlessly, no blink or anything
3710X, 7.6.0 build 4120-28
My Roku 2XS working flawlessly, no blink
3100X, 7.6.0 build 4125-02
My Roku TV wouldn't take screenshots at all, no luck! I tried jpg and png, nothing. I didn't get any other side effects, just no pic (and no single pause in-game like all the rest did when taking a pic)
5120X, 7.6.0 build 4127-08
Untested - old Roku Stick