The effect of experimental methodology on the Just Noticeable Difference (JND) of the black level (BL) is assessed using a set of representative natural images. Two methodologies, known in psychophysics to determine JNDs, are mutually compared: an adapted method of adjustment with a same-difference task and a one-up-two-down staircase method with a two-alternative forced-choice task. Thus far, the literature has shown contradictory results for which of the two methods yields the lower difference thresholds. Experimental results show that the second method yields lower JNDs, but is slightly less efficient. Based on this second method, it was found that the JND of the BL for natural images ranges from 0.2 to 1.0 cd/m2, varying with image content significantly. © Copyright 2009 Society for Information Display.