【Objective】Soil color is an important soil property. It is frequently used by soil scientists for the identification and classification of soil. It is also used as an indicator of many soil properties. Soil organic matter(SOM)is the most important pigment, that colors the soil in black color.【Method】In this study, a total of 30 mollisol samples were collected from the typical black soil region of northeast China. The SOM was physically separated into four fractions: light fraction, coarse particle fraction, fine particle fraction, and mineral‐associated fraction. Based on the CIE L*a*b* color analysis system, the darkness of bulk soil and soil physical fractions were analyzed using a spectro-colorimeter. The objective was to quantify the relationships between SOM content and the color of black soil and to reveal the contribution of different physical fractions to soil blackness.【Result】Bulk soil blackness was strongly positively correlated with the SOM content. Similarly, significant relationships were observed between the blackness of physical fractions and the organic carbon content in corresponding fractions. This relationship was gradually strengthened with the increase in the stability of the fractions. As for the color of soil physical fractions, the blackness value of light and coarse particle fractions was greater than that of fine particle and mineral‐associated fractions. Also, correlation analysis showed that there was no significant relationship between the blackness of light or coarse particle fractions and the bulk soil blackness, and the contribution rate of the two fractions to bulk soil blackness was only 2.6%.【Conclusion】The the mineral‐associated fractions, as the main storage location of soil humus, contributed more than 81% to bulk soil blackness and plays a decisive role in coloring the soil black.