Why does projecting black light from a screen mask white light shining through it? I had a Macbook Pro, and remember being intrigued when I noticed that the apple logo on the back of the screen allowed sunlight to shine through in an Apple shape on the front of the screen when turned off. But when the screen was turned on, it would go black. The black screen eliminated the apple shape that was visible when the screen was not lit.
I'm not quite sure I understand how black light works (and I assume black light is emitted, to achieve perfectly black color, darker than the screen itself), much less why it would mask white light shining through the emission surface. It seems like the white light would show through.
So. Why does projecting black light from a screen mask white light shining through it?
 A: Macbooks use LCD screens. These have a liquid crystal layer with a backlight behind it. The LCD screen works by changing its opacity i.e. it controls the amount of light that can be transmitted through it from the backlight. If the LCD is not blocking any transmitted light then all the light from the backlight is transmitted and the screen looks white. If the LCD is blocking all the transmitted light then it lets no light through and looks black.
And this explains why a black screen blocks sunlight shining through the lid. It's because a black screen blocks all light.
A: Perhaps it is working as a grease spot photometer as well.  The display back light may just be much brighter than the diffuse light coming through the logo.
http://www.phy6.org/outreach/edu/greaspot.htm 
As mentioned in the other answer it is possible that the LCD is transmitting less light when it is operating and so blocking out the back light (and the apple light)
To test which one is responsible it might be possible to test this by shining a bright light (much brighter than the back light) into the apple and seeing how a white or black area of the LCD passes this light to determine which effect is more pronounced.
