When using picosecond lasers, black patterns of varying contrasts are obtained on the surface of aluminum alloys by changing the laser marking parameters. In this study, the surface of experimental marking samples is measured using a laser scanning microscope, and the mechanism of picosecond laser blackening is analyzed through the theoretical modeling of electromagnetic waves. Consequently, a dense peak-valley morphology with different maximum height values on the micro-nano scale is formed on the surface of the laser marking samples. The results indicate that a larger maximum height of the micro-nano peak-valley morphology on the sample surface causes a lower surface reflectivity and the formation of black patterns with higher contrasts. Additionally, the contrast obtained using the sample gray value is almost equal to that obtained via theoretical modeling, which proves that the above phenomenon is the main reason for the laser blackening of aluminum alloys.