How do you get the color black on computer and television screens?

to Understand this you must first understand how it makes all the other colours.

When a color TV needs to create a red dot (pixel), it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously -- the three colors mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colors on a TV screen are combinations of red, green and blue. So it's basicly just a lack of light.