Thanks to the outstanding Nando Times for this piece of info:
Black is "K" in CMYK color notation to avoid confusion with blue and brown, which are keyed as, respectively, U and N.
Actually, says "Dan", K stands for key. It was traditionally the reference color used to register the other process colors in printing. In inkjet printing, the proper use of black is still fundamentally important for getting good color reproduction.
TechTarget also says:
CMYK is a scheme for combining primary pigments. The C stands for cyan (aqua), M stands for magenta (pink), Y is yellow, and K stands for black. The CMYK pigment model works like an "upside-down" version of the RGB (red, green, and blue) color model. Many paint and draw programs can make use of either the RGB or the CMYK model. The RGB scheme is used mainly for computer displays, while the CMYK model is used for printed color illustrations (hard copy). More
Updated November 21, 2002