Automatic evaluation of X-ray weld image quality is an important foundation for automatic evaluation of weld image defects. A digital blackness meter model was proposed to realize the automatic evaluation of X-ray weld image quality. Firstly, in order to obtain the physical blackness by numerical calculation, the physical illumination model and the weld blackness model were combined by the digital blackness meter model. Then, through the analysis of the correlation between the physical blackness value of the sample image and the corresponding grayscale value, a method for obtaining the parameters of the digital blackness meter model was given. Finally, an X-ray weld film blackness automatic evaluation algorithm was proposed. The experiments on the actual X-ray weld images show that, the accuracy of the proposed algorithm can reach 99% without manual intervention. The cross-validation experiments show that the sensitivity of the proposed method is 98.5% and the specificity of the method can reach 100%. The digital blackness meter model based on illumination model and blackness model as well as the solving algorithm can replace the commonly used physical blackness meter and realize the automation of weld image quality evaluation.