Just tried rebooting, while the system was set to "extend displays" and one of the displays was dark and the other visible.

On boot up, the "visible" side is still visible and it is set to "show desktop on monitor x".
And the dark side monitor has the power icon flash, which shows it is in standby and awaiting a signal (the same as the main monitor, if I shut the PC down but don't power off the monitor).

I'll see if I have any older video cards around; I usually pass my stuff on to friends or relatives when I upgrade.
I suppose even a really old card that would not box well, could show if the card is the issue.

It could easily be the video card; anything I buy from my local store has a four year "instant product replacement" warranty.
If the card is defective, I'll get a brand new one at no cost, or the next card up if my card is not available.