It is down to how our eyes perceive black and white.
It works by “subtracting” colours from the light. So, if you were to mix Cyan and Yellow you would get green, however if you remove green (and the subsequent mixed colours yellow and cyan) you are left with Magenta.
So how do you produce black? This is done by combining the three colours, Cyan, Magenta and Yellow. So, for the printing process a Black (or Keystone) colour is used to save on printing costs. This process is called the CMYK process. The standard process for printing physical items, for example, photographs. Starting with the white paper, adding colours to make darker pigments.
You may have also of heard RGB, generally seen in conjunction with televisions and computers. This is a process of adding colours together to make others, for example adding a mix of Red and Blue will make Magenta. This is called the Additive colour model. Starting with a black screen, adding colours to change pigments, ending up with white, a mix of every colour on the spectrum.
Both a simple exploit of our natural sense.
