When heavily oxidized porous silicon is stored in ambient air for prolonged periods, a broad blue photoluminescence band with nanosecond decay times gradually appears. Not only PL intensity, but peak position, half-width and temperature dependence evolve with material aging. Very similar photoluminescence is shown to be characteristic of many high surface area materials stored in air for months. We stress here that hydrocarbon contamination can occur during air exposure, in addition to hydration by water vapour. Preliminary Fourier transform infrared spectroscopy and secondary ion mass spectrometry studies, together with published data on blue-emitting porous alumina and organics, suggest that the gradual build-up of low concentrations of carbonyl-related chromophores could give rise to the emergence of blue-green luminescence in aged oxidized porous Si structures.