Was "Istanbul" not commonly known, and if not, when did that change?
"Istanbul", a slang term roughly meaning "the city", was not really an official name for the city for most of Ottoman history. The Turkish word "Konstantiniye" was used alongside many other names, and one of the names used most frequently in a formal context. This, however, changed when Atatürk officially renamed the city to Istanbul in the 1920s, which slowly became predominant in most western languages (still not Greek, for example) over the following decades.