The CMYK color model began practical use in the 1890s when newspapers started publishing color comic strips. This period marked the transition to a subtractive color model that relies on inks masking colors on white paper.
The K in CMYK represents the keyline plate which often contained the outline of solid areas and fine details like serifs. Black ink absorbs more light and yields much better blacks than any other mixture compared to combining cyan, magenta, and yellow.
Halftoning creates lighter colors by printing tiny dots in specific patterns to achieve less than full saturation of primary colors. Magenta printed with a 20% halftone produces pink because the human eye perceives the tiny dots on large white paper as lighter and less saturated.
CMYK printers have a relatively small color gamut because standard inks cannot create light or highly saturated colors found on electronic displays. Processes such as Pantone's Hexachrome add light cyan and magenta inks to expand the available range for desktop inkjet printers.
A composite black uses only cyan, magenta, and yellow inks but results in weaker blacks that soak the paper and cause bleeding. Rich black involves applying a colored or gray CMY bedding first before adding a full black layer to make a deep, dark area.