I was curious too and found this: "Objects and people in its path shielded objects behind them by absorbing the light and energy. The surrounding light bleached the concrete or stone around the "shadow." In other words, those eerie shadows are actually how the sidewalk or building looked, more or less, before the nuclear blast." Source
Zqin t1_j94v2mn wrote
Reply to comment by humanbeing2018 in Nuclear shadow, Nagasaki by allez05
I was curious too and found this: "Objects and people in its path shielded objects behind them by absorbing the light and energy. The surrounding light bleached the concrete or stone around the "shadow." In other words, those eerie shadows are actually how the sidewalk or building looked, more or less, before the nuclear blast." Source