Obviously the sky wasn't night-black. And the day wasn't an outlier. Although dull for September, it would have been moderately bright for June. We might speak of the sky being black, meaning very overcast, if it had suddenly darkened. We say black thunder clouds, which aren't. The BOM solar exposure figures are totals for the day, and there may have been periods which were significantly darker than others. Allowing for colloquiality, exaggeration, and perhaps an emotional need to talk about her own experience of the day, I do think NC was referring to the weather and not to a psychological or physiological event.