They were expecting air raids, but the Enola Gay came with only two other bombers. You never saw three planes coming in for an attack -- what would be the point?
"The day was very hot, beautiful blue sky. No clouds. I saw a silver airplane with a white long tail. It looked pretty. I said to my classmates, 'look look over there!' ... I was a 13-year-old girl, I'd never seen a bomb before. I looked up and pointed, and before my hand was down ... I saw the airplane dropping a white thing."
U.S. Air Force
There were only two colors in the 1940s, so it was bound to be either white or black.
Then she described something like a "strong wind," but added that the word "wind" didn't properly describe the sheer force of the atomic blast. Nuclear detonations happen fast, and if you're anywhere near one, you'll be killed way before you have a chance to see that iconic mushroom cloud, much less cling to a chain link fence while a playground dissolves in fire. Most of the people who died in the blast had the blood in their brains evaporate before they could realize there'd been a detonation.
The Japanese government was as shocked as the victims on the ground. The EMP cut out all communications from Hiroshima, and since there'd been no visible massive force of bombers flying over the city, they assumed it was some kind of weird technical fluke. Sure, there were rumors of a horrific explosion, but they figured those were wild exaggerations. And in fairness to them, "one bomb leveling an entire city" was the sort of thing that sounded like sci-fi nonsense, right up until someone figured out how to make it happen.
Some things are better off staying science fiction.