I am also wondering about this for years now, as black bags heats up significantly in the sun... i havn`t found out why most bags are black, but i made up a theory:
When photography came up as a wider spread phenomenon there weren`t synthetic materials invented yet, so camera bags were made out of leather for the most. Leather is pretty resilient, therefore a good choice for a camera bag, but untreated/uncolored leather doesn`t have a very nice color. So leather often is colored brown or black to make it look better/more uniform, as uncolored leather can show differences in color or structure.
A black leather bag does look better and even can look somewhat elegant if the black leather is shiny.
Apart from the looks a camera bag also should be water-repellent, you can get leather water-repellent if you grease it - with ordinary shoe-shine for example. Quite some shoe-shines have black color, to re-color a black shoe having lost some of its color due to wear - and if your camera bag is blackened leather you can use just shoe-shine to care for the leather and to make the bag water-repellent again. You don't need any special care-product and back in those days a lot of people had shoe-shine at home.
But it also could have been some sort of fashion. Gramophones are black for the most, instrument cases (cellos, violins etc.) cases for binoculars... though at least gramophones aren`t covered in leather for the most but cloth.
This could be the reason(s) why camera bags were made black in the beginning - and then they just kept on doing it, because, you know, a camera bag is black, right?