Soil color is an important physical property for studying the genesis, formation environment and fertility of the soil. Generally, the soil with a higher organic matter content appears darker, but the soil color of the Calci-Aquic (CA) Vertisols with low organic matter contents also appear very black. The results based on the traditional chemical extraction method of black matters indicated that the combination of highly aromatized humus with soil particles makes the CA Vertisols appearing black. However, the chemical extraction method only partially extracted the humus ingredients, and what is more important, it destroyed the structure of organic matter and organic-inorganic complexes. The study on the black matter extracted from the CA Vertisols by the physical extraction method without destroying structures of organic-inorganic complexes showed that the black organic-inorganic complexes formed from smectites adsorbing organic matters were the decisive factor of the CA Vertisols apprearing black. However, the effects of the structures of the organic matters absorbed by the smectites on the color of the black organic-inorganic complexes remains unclear. Therefore, this study aims to explore the mechanism of these organic-inorganic complexes appearing black from the structural characteristics of the organic matters in the black organic-inorganic complexes and their relationships to smectites. Firstly, the physical method was adopted to extract the light-colored components (>53 μm particles, white (W) and light white (LW))) and black components (light black (LB), black (B) and nano black (NB)) from the topsoil layer (0～40 cm) of the typical CA Vertisols in three sites of the Huang-Huai-Hai Plain in China. The blackness, organic matter structures and smectite contents of the extracted components were then measured by Spectrophotometer, solid-state 13C NMR spectrometer and X-ray diffractometer. Finally, correlation analysis and path analysis of smectites, organic matter structures and soil blackness showed that the direct and indirect effects through alkyl C, carboxyl C, amino C and O-alkyl C on soil blackness are strong in all three soils, while both of the direct and indirect effects of aromatic C content and aromaticity on soil blackness are weak. Besides, the direct and the indirect effects through carboxyl C, amino C and alkyl C of smectites on soil blackness are also strong. Therefore, it is the carboxyl C, amino C, alkyl C and O-alkyl C, rather than the aromatic carbon and high aromaticity in the traditional view, that determine the black color of the CA Vertisols were by being selectively absorbed by the widely existing smectites in the CA Vertisols to form organic-inorganic complexes.