what does "the blow fell."mean?
Pierre Salinger had remained at his post in the communications shack, listening with one ear to the subdued Cabinet debate over whether or not to obey Stranger's order to fly nonstop to Andrews, by-passing Love Field, and, with the other ear, awaiting fresh word from Stranger, who had been droning his broken-record "Stand by . . . Stand by . . . Stand by" every fifteen seconds. Then the blow fell. Abruptly the monotonous pitch changed. It rose: "Situation to Wayside. The President is dead. Repeat, the President is dead."

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"The blow fell" means something bad "finally" happened in this particular circumstance. Salinger had been in a "holding pattern" for a while, and finally the bad news arrived. In other words, think of someone holding a bat or a weapon, ready to strike. That person holds that position for a while, and finally, he strikes - or, "the blow falls." (PS a "blow" here means a strike or a hit, not a movement of the wind or air).