Blocking artifact is the most prevailing degradation caused by block-based DCT coding techniques under low bit-rate conditions. To alleviate blockings perceptually, it is desirable to measure the visibility of blocking artifacts. In this paper, we propose an efficient method of estimating the visual sensitivity of blocking artifacts in block-based DCT coding. The differences on block boundaries are measured and transformed into block discontinuity map. We consider the effects of luminance adaptation and texture masking on the blockings and integrate them using nonlinear operator to form an overall masking map. This masking map is then incorporated with the discontinuity map to generate the Blocking Visual Sensitivity Map (BVSM). This map can be used to guide perceptual quality assessment, codec parameter optimization, post-processing, etc. We demonstrate the validity of the BVSM through its application in image quality assessment.