Chubb et al. [Journal of the Optical Society of America A 11 (1994) 2350] investigated preattentive discrimination of achromatic textures comprising random mixtures of 17 Weber contrasts ranging linearly from -1 to 1. They showed that only a single mechanism B is used to discriminate between textures whose histograms are equated in mean and in variance. Although they provided a partial characterization of B, their methods did not allow them to measure the sensitivity of B to texture mean and variance. Here, additional measurements are performed to complete the functional characterization of B. The results reveal that B (i) is strongly activated by texture elements of the lowest contrast (near -1), (ii) is slightly activated by texture elements of contrast -0.875, and (iii) barely distinguishes the 15 contrasts ranging from -0.75 all the way up to 1. To reflect the sharpness of its tuning to very dark, sparse elements in a predominantly bright scene, we call B the blackshot mechanism.