Saturday 4th January was truly an extroadinary and terrible day for the south-east corner of Australia. Skies turned pitch black at around 3-5 pm, when normally these areas would be in daylight. I have never been aware of this sort of phenomenon. Is it possible that thick smoke alone could have caused this, or was it larger particles such as burnt leaves and bark, picked up by strong updraughts from pyrocumulus clouds? It is worth noting that this occurred right before a cold front passed through the area, bringing southerly winds, and that pyrocumulus clouds had formed, and lightning was being generated from them.
It's a combination of both the density and amount of smoke in the air and ash produced by the fire.
The thing about cold fronts and bush fires is they make the conditions for fire worse by pushing hot air in front of them. They have been blamed for the severity of the bush fires in Victoria in 2009, 1983 and 1939.
It was a combination of thick smoke and pyrocumulus clouds. Possibly there might have been other types of cloud as well, Ash rising on the updraught may also have been a factor. I am informed that normal types of cloud sometimes made an appearance during the drought. Near-pitch darkness in daylight hours is also characteristic of volcanic eruptions, but I am not suggesting that volcanic eruptions were a factor in this case, though they make an analogy.
During cumulonimbus events in England, usually accompanied by rain and thunder, we often get very dim light in mid afternoon, long before evening time, though I wouldn't describe it as pitch darkness but more a sort of twilight. If enormous plumes of smoke were added, it very likely would have become almost pitch dark.
If you think how white smoke looks when it is rising elsewhere and you are looking at sunlight reflecting off it ... what you are then seeing, is some of the light that the folks who have that smoke between them and the sun are not getting. Smoke will also absorb a lot of sunlight (and re-radiate it as infra-red), especially if it's sooty black smoke rather than whitish ash.
On a gloomy note, you are experiencing what a nuclear war would cause on a global scale (a nuclear winter, caused by the ashes of civilisation). At least your smoke is not radioactive and relatively short-term ....