Some info
An SLR camera has a mirror sitting behind the lens just in front of the sensor that reflects the image from the lens through a Pentaprism to the view finder, so, what you see in the viewfinder is what you get. (If light was also split to the LCD then the image in the viewfinder and LCD would be 50% bright on each display)
As you press the shutter release button the mirror flips up out of the way, at the same time the shutter mechanism opens to whatever you set the shutter speed to – 1/4000sec to 3mins+ on the 10D. Whilst the shutter is open the view finder 'blacks out' because the mirror is flipped up out the way so not light is entering the viewfinder. Once the photo is taken it’s displayed on the LCD.