I've observed this too -- it happens on one of my laptops every couple of days, or so. I've always had the impression that, for some reason, this was the screensaver kicking in. They way that happened always gave me that idea.
I'm now pretty sure that, sometimes, for some reason the screensaver kicks in even though the system is not idle. I just finished typing, and began reading something on the screen, when the screen went black about three seconds after I stopped typing.
I did nothing, and the screen continued to stay black. After waiting about ten seconds, I pressed the shift key, and my desktop came back.
It's the screensaver. Something's causing it to trip, when it shouldn't.
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