High-temperature black bodies used as primary standards of spectral irradiance have to meet several requirements. One important demand is that the spectral irradiance of the black body is uniquely determined by Planck's radiation law, where the (radiometric) temperature of the black body is the only parameter that determines its relative spectral distribution. By combining determinations of the radiometric temperature of a black body over a wide temperature range with the corresponding spectral measurements, black-body spectra for different temperatures can be compared. This allows the use of self-consistency checks to ascertain whether (i) the temperature determinations are coherent over a wide temperature range; and (ii) the different spectral distributions show any deviation from Planck's law. Using this method of self-consistent calibrations, new absorption bands in the spectrum of a black body were discovered at very high temperatures. The level of absorption increases with temperature and depends on the operating conditions and imperfections of the black-body system. Possible origins of this effect are discussed and modifications proposed to avoid such absorption bands.