Through a little research on Wikipedia, I found out that there are a few reasons why black ink is used alongside CMY.... rather than just combining the three to make black. Firstly, the majority of text is printed in black and therefore requires fine detail... in order to do this with overlaying three inks, each screen would have to line up perfectly(the registration marks would have to align) to avoid blurring. Secondly, to produce black from cmy, the paper would be pooled with ink. This would take forever to dry and this isn't always viable. Lastly, the colour produced by mixing cmy isn't always a decent black... often a crappy brown colour, which is called a "composite black"
CYMK is an example of what you'd call subtractive colour, a process where paints, films and inks etc are mixed to create colour. The process starts with white light and CMY remove wavelengths from the white creating colour.... apparently... and the more layers you add the darker the colour. This is because less light is being reflected. If you mix CMY together you end up with the crappy muddy brown colour outlined above
