PIE * bhel(燃やす、輝く)から。語源はblank、blueと同じ。 古英語のblackは曖昧な単語で、黒と白、あるいは他の色の両方を指す。
For years it has been a common practice to use red ink instead of black in showing a loss or deficit on corporate books, but not until the heavy losses of 1921 did the contrast in colors come to have a widely understood meaning. ["Saturday Evening Post," July 22, 1922]