Rich black is the name given to a special trick in the creation of black colour in print media when using process colours. (and no, it has nothing to do with the monetary situation of people with a dark skin colour)

Why on earth...?

Have you ever noticed - primarily in magazines - that things you thought should look like completely black actually don't? It's true, and many people have noticed that. So many, in fact, that someone came up with a bright solution; which is where "Rich Black" comes in.

The problem

Usually - when printing black text etc - the ink rollers can go through the ink 3-4 times before the ink is needed. This saturates the rollers with ink, resulting in a pure black colour. When large areas of paper have to be covered with ink, the rollers will will be depleted on every single revolution. This results in less ink ending up on the pages, resulting in a grey-ish, dull black colour. We don't like that.

The simple solution

The solution to the problem is as simple as it is clever. Say, when the rollers have passed over the paper 3-4 revolutions, they are at their minimum of ink capacity, and you might be "missing" about 10 % pigment to get a completely black area.

Initially, printers solved the problem by running another round of black over the same areas. This was a good solution, except the colours have to be lined up EXACTLY. A quarter of a millimeter misalignment either way can cause problems, as there will be a very slight difference on the edges of the black areas. The eye will perceive this as being blur, making the page look sloppy, messy and unprofessional. Also, this meant that a process colour print machine would have to have 5 rollers (CMYK+K) instead of the normal four. This also means extra time aligning all the rolls - anyone who has worked in a print shop knows how abhorrently annoying this is; Hell, two colours can be bad enough some times, let alone five.

So; Some bright soul thought up a brilliant idea; Would it really matter what colour we add to the black? The answer was no, and it was discovered that adding 40% cyan to the mix (this means that black = 40C0M0Y100K). This formula results in a slightly greenish black (the colour looks black, but will have greenish reflections). However: a greenish black is preferrable to a grey colour. This effectively solved the problem.