[Objective] Color of a soil is an important physical indicator characterizing genesis, classification and fertility of the soil. Soils black in color are generally high in organic matter content, but the Shajiang black soil (Calci-Aquic (SCA) Vertisols) in China is a typical soil, dark in color but low in organic matter content. This study aims to establish an effective method for extracting the black color matter from the SCA Vertisols, to explore their compositions and characteristics, so as to expose how organic matter forms and accumulates in the soil. [Methods] In this paper, soil samples were collected from the topsoil layers (0-40 cm) and black soil layers (40-70 cm) of three typical SCA Vertisols of the Huang-Huai-Hai Plain in China and processed with three physical methods, separately, for fractionation by color. The three physical methods includes 1) (HP) to treat prepare the sample with hexametaphosphate (HMP) and have it subjected to oscillation and multi-level sonication; 2) (SC) to treat the sample with sodium carbonate solution and have it subjected to oscillation and multi-level sonication; and 3)(US)to treat the sample with ultrasound only for dispersion. And for fractionation by color, six fractions, that is, POM, White and Light White (white group) and Light Black, Black and Deep black (dark group), were set up. After the soil sample was fractionated, each fraction was analyzed for blackness, organic matter content, particle size distribution and mineral composition with a spectrophotometer, TOC instrument, nano-laser particle size analyzer and X-ray diffractometer separately. [Results] Blackness of the soil had nothing to do with organic matter content, regardless of treatment method, but much with amount of anyone of the dark fractions (Light Black, Black and Deep black). Among the three methods, the HP method got the largest dark fractions, which were significantly related to blackness (P<0.01) of the soil. Particles of organic-inorganic complexes <100 nm amounted to more than 50% of the total dark fractions (TDF), and composed mostly of Fraction DB and B, accounting for over 90%. The content of <100nm organic-inorganic complexes was significantly and positively related to blackness (P<0.01) of the soil, while the content of >100 nm organic-inorganic complexes was negatively related, in all the soil samples, though from different sampling sites. Particles of organic-inorganic complexes <100 nm amounted to 18.4-60.4 g·kg^-1 in the black soil layer, significantly higher than that (2.4-15.2 g·kg^-1) in the topsoil layer in all the three SCA vertisols, and content of organic-inorganic complexes <100 nm in particle size was also significantly and positively related to blackness (R²=0.971) of the soil.[Conclusion] The HP method is the best one for extracting fractionation of SCA Vertisols by color. Soil color depends mainly on content of black nano-organic-inorganic complexes formed out of adsorption of organic matter by smectite in the SCA Vertisols rather than organic matter content, which has relatively less direct impact.