A rough setting for black level can be found by making the underscanned margins as black as possible. Using black picture content, instead of the margins, always assures an accurate setting. Ask a service technician to repair this problem. In a television studio environment there are standards for absolute luminance. Resist the temptation to set your monitor too bright.
Excessive brightness has a number of disadvantages.
First, your sensitivity to flicker increases as brightness increases, so setting your monitor too bright is likely to increase your perception of flicker. Second, a number of phenomena act to scatter light onto the face of the screen, and the higher the brightness of bright areas of the picture, the more light is scattered into the dark areas. Third, operation at high brightness tends to defocus the electron beam of the CRT, resulting in poor sharpness. It may even be necessary to iterate between the two controls a few times in order to set the combination that both reproduces black correctly and reproduces white at the brightness you desire. A Technical Introduction to Digital Video .