The surface of the anodized film had uneven color(blacking).In order to find out the reason，macroscopic analysis, fracture observation and other physical and chemical analysies were carried out.It is determined that the reason for the blackening of the anodized film is due to the existence of composition segregation (high alloy content),resulting in a relatively smaller grain size,worse corrosion resistance and difference of microstructure of film.There are more alloying compounds inside the film, therefore the film exhibits a black color.