According to various websites - The term was first used in WWII and meant one of the RAF's huge 4000lb bombs that could obliterate a whole block in one explosion. After the war, advertisers picked it up and used the term to mean anything that caused a sensation or 'explosion'.
The first time the word was used in print in the sense you mean was in a book called 'Friends', published in 1957. The author, G Smith, wrote: "One day I had what seemed to me like a blockbuster of an idea for a musical play." So, even then, it referred to some form of entertainment production.