During my senior year of high school English, we watched several video lectures of a Duke University professor of whom I can't remember the name. He explained, among other things, that back during Elizabethan theatre there were no tickets to shows. Instead an usher or doorman stood around with a locked box, with a hole in the top, and would collect payment for the shows. The box office was a small room in back were all the additional boxes where held, and where all the accounting was conducted for the theatre.
A blockbuster occurred when there was so much money coming in that the boxes or blocks, were busting with money, and frantic ushers were running back and forth to the box office trying to replace them.