A study focused on the implication of the blue and black colors in Korean traditional costume is presented. The presented study shows that the blue color in most of literatures regarding Korean traditional costume implies not only green and blue colors, but also dark blue and black colors. In a broad sense, the blue color implies a color category which includes green, indigo, blue, dark blue and black. On the other hand, the color includes blue, dark blue and black in a narrow sense. The rotor black, however, should not be interpreted as blue and dark blue because it has two bypes of black colors with and without reddish tone. Four reasons that the color blue was understood as the color black were discussed in this paper. The fundamental reason was that the blue color looked like black because of dark shadows. Secondly, there was a difference of color awareness between the color blue in Korea and in China. Thirdly, there was a color difference related to the type of textiles. Finally, other substitutes were used because of the lack of blue dyes.