Layers of Nd-doped lead zirconate titanate (PZT) with Ag/Pd electrodes were stressed by DC voltage at about 500 degrees C. Under field, a black "color" front, visible in dark-field optical microscopy, moved from the cathode into the PZT. The PZT layer became highly conducting after some time but degradation was localized to a near-surface zone and could be removed mechanically, leaving an insulating core bulk zone of PZT with the color front still visible. The time-dependent motion of this color front and the electrical properties of the insulating PZT core bulk zone were investigated upon field and during subsequent relaxation. The results suggest that a redistribution of lead vacancies and built-up of space charge zones at grain boundaries with oxygen vacancy accumulation cause the blackening and conductivity changes. Density of states (DOS) and optical absorption spectra were obtained from density functional theory (DFT) based calculations for several defects in PZT and show that blackening is expected for an increased oxygen vacancy concentration. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).