How does a screen (computer moniter movie theater et cetera) produce the color black? This might seem a dumb question, but here goes: 
I can't help but notice that a black screen on my computer looks more like black than the screen when the computer is turned off. Similarly a black screen at the movie theater certainly appears darker than the screen prior to the projector being turned on.
Now, insofar as I'm aware, there isn't a combination of light that does this, so I'm thinking it's an illusion involving contrast? I thought it was interesting. Sorry, there don't seem to be tags appropriate to this.
 A: There is no black light. Imaging devices control the color strength by the power they give to the pixels. In case of the black color (i.e. (0,0,0) on the RGB scale), they don't give any power to the given screen area.
If the imaging device is powered off, it is surely black. But, if it is powered on, a little bit of light may come even from the black areas: for example, some LCD monitors have a background lighting and a little bit of it can pass even the black pixels on them. In the case of projectors the situation is similar.
Thus, the black area on the screens (if the display is turned on) are surely lighter, at least a little bit, as if the device would be turned off.
What you experience, was probably an optical illusion. Our eyes are adaptive: our retina (not the Apple bulls*, but the real retina, in our eyes) reconfigures itself if there is too many light or too few. Essentially it can "pull up" or "pull down" a defense layer in the cells containing the rodophsin. This is why we can see exactly well in a closed room lighted by bulbs, and below the free sky on the Sun. In the reality, the light produced by the Sun has around 100 times more power, as we see in the light bulbs, but we see in both cases only that "there is light".
Also this is why if we feel a little bit annoying and so strong light for a short time, if we go out. Our eyes require a little bit of time for the pullup.
As you see the monitor, which is most cases full with strong colors (for example, since some decades, most of the desktops use mainly dark characters on white background), your eye accepts that light intensity. Thus, if you see the a little black area, it happens with your white-adapted retina. This is why you see it darker.
A little experiment: don't turn off your monitor, but make it black (for example, by setting the background color to black). Power off every light in your room (if there is now night by you). You will see, that even the screen is black, it gives at least a little bit of light.
Now turn off the monitor. You will see, that now it is really black. :-)
In short, you compare the light strength of full-screen black display in a dark room, to the same monitor, but powered off.
