The buildings in which they store the barrels are black at the bottom (two to six feet) from the stuff as well.
But I was surprised to see so much of it in residential neighborhoods.
Interesting observation ablut 2-6 feet. A Freeper told us that CO2 was heavier than air and would sink. Your observation validates that thought
Further, perhaps, the fungas eats the CO2 and becomes black with the carbon residue.
I have always believed that if the bad guys were serious about carbon emissions they would go after carbonated beverages including cokes and Budweiser. All the fizz in cokes and beer is atmospheric CO2 emission, even after injested.