Dayflower, knotgrass (indigo) and Prussian blue are known to be blue colorants used in traditional ukiyo-e color prints. For the non-destructive determination of the three blue colorants, the visible near-infrared reflection spectrum of each standard color sample was measured with a portable spectrophotometer using fiber optics. The three spectra, corresponding to each colorant, showed different patterns in the range of 630∼900 nm, and the three colorants could be easily identified by each respective spectrum pattern. This analytical method was applied for the identification of blue colorant(s) used in 36 prints of "Thirty-six Views of Mt. Fuji", the key-blocks of which were printed with blue color by Katsusika Hokusai, published in ca. 1830∼33. It was identified that every key-block of the 36 prints was printed with indigo, which was hitherto assumed to be Prussian blue, while all color-blocks were printed with Prussian blue. This pattern of color usage, indigo for the key-block and Prussian blue for the color-blocks, was also observed in 5 prints of the "Shokoku Taki Meguri" series, the key-blocks of which were printed with blue color, by Katsusika Hokuai, published in ca. 1832∼33.