In article <3F08052E.DB23CA3D@yahoo.co.jp>
kazhagiwara@yahoo.co.jp writes:

  > CMYKの"K"なんですが、それぞれ、一般的にはシアン、マジェンタ、
  > イエロー、黒、と説明されているようです。たとえば、これ。
  > http://www.toyoshingo.co.jp/yokoin/home%20page/insatsu-colum/ic-cmykmagic.htm
  > 
  > でも、"K"というのは本当に日本語のKuroなんでしょうか。私にはほか
  > に語源も思いつきませんが、これだけ日本語というのも……。

調べてみますと、「K」は「Key」からきてるようです。

Googleでも見つけられますよ。

--- http://www.cknow.com/ckinfo/acro_c/cmyk_1.shtml
a yellow banana has no color of its own (i.e., it does not broadcast the color yellow); colorants in the skin absorb all wavelengths except those producing yellow. While black could be obtained by mixing the three secondary colors (cyan, magenta, and yellow), in printing it is usually produced using a black ink. The "K" in black is used to prevent confusion with the "B" in RGB (and the black color is often also called a "key" color). 
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--- http://www.color-printing.com/inkjet-cmyk.html
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. Black is used to reproduce text and line art, neutralize the contamination of CMY inks, add density and reduce total ink consumption in wide-format inkjet printing.
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こんな感じです。
ほり ＠moat.net