Optical distortions arising from inhomogeneous air in the telescope dome can be a non-negligible contributor to the delivered image quality (DIQ). Optimization of the dome environment is particularly important for first-class wide-angle imagers, such as the DECAM on the 4-m Blanco telescope, Omegacam at CFHT and, in the future, LSST. However, the standard method of comparing the DIQ with the site seeing does not single out the effect of the dome, being affected by contributions from other sources and by biases. We developed the dome seeing monitor for the Blanco telescope. It propagates the collimated 4-cm beam from the mirror cell to the top ring and back, parallel to the main beam. The angle-of-arrival fluctuations on this 10 m long path gives a quantitative estimate of the dome seeing. We describe the instrument and its software. The results for the Blanco telescope are presented. The median dome seeing is around 0:2 ''. It shows the expected dependence on the temperature difference with the outside air and on the speed and direction of the wind.