Usually when you notice a sudden flash of light it's because of its brightness. You notice lightning because it brightens the sky. You notice a light in a room because it makes the room brighter.
And, yes, when they go out you notice that, too. But it's typically the suddenness of the darkness that gets your attention. In a "faster than the speed of dark" kind of way.
Tonight, though, I have been sitting in the living room and looking out at the street as the evening turns darker and the sky goes from cloudy grey to blue and all of the green trees move into silhouette. And as I've been sitting here I realized that I thought it had been brighter in one section of the silhouetted trees.
Not that it had been "light," mind you, but in my mind's eye I was pretty sure I had seen it somehow brighter.
It was the absence of light in one particular area of the darkness that caught my attention. Not suddenly, but kind of gradually. Like a dawning of realization - if the dawning were dark, instead of light.
A truck turned the corner and headed down the street as I was looking out the window, and in the middle of the dark mass of the tree a streetlight flickered on. I read somewhere that this can happen if the wiring is old. The vibrations caused by passing vehicles can make the lights turn on - and off.
It was only on for about 30 seconds - barely enough time to color-shift as it warmed up - and then it was out again. And that mass of trees seemed even darker than before.
The speed of dark. It may be gradual, but it's definitive.