Rich black, used in offset printing, is a combination of black ink overprinting tints of one or more CMY colors in order to improve density of bold headlines or black backgrounds. Generally, small areas of black (less than an 1/8" wide) appear sufficiently opaque without the addition of rich black. Do not use rich black for small text.
Black is a component of the CMYK ink combination used to reproduce photographs. Hence, in an effort to lay down extra black on a page, a printer runs the risk of darkening photos and other multi-color graphics.
A rich black combination can be as simple as 40 percent cyan plus 100 percent black (expressed 40c, 100k). However, surrounding colors can determine tints used for rich black. For example, if yellow text is knocked out of a black background, a tint of yellow minimizes possible color leaks around the edges. If the yellow contains a hit of magenta (perhaps 15 percent), it may be advantageous to include this in the rich black as well. The background may then combine 40c, 15m, 15y, 100k.
In another example, a bold black headline may be used against the clouds of a light sky blue. In the photo there are negligible amounts of all four process colors, though cyan is most prevalent. In this case, or against a simple white background, 40c, 100k is fine.
When separating colors, professional software makes allowances for rich black by comparing the background to determine whether it is necessary to shrink underlying tints slightly from edges. Where this is advantageous on a white background, it may not be on the aforementioned examples.
It is not necessary to mix rich black for inkjet printing. Ink absorption into paper usually provides sufficient density. Printers with an excess of eight colors include two different inks, called matte black and photo black. The matte black, with higher pigment concentration, is used for solid areas, and the more translucent photo black is used for photographs. The 12-ink printer at Reactive Imaging combines matte black, photo black, and two separate grays to reproduce grayscale photographs in quadtone.