Across the promenade from The Blue Mosque is The Hagia Sophia. Visiting it is a bit of a different experience. Think of it like traveling through an airport security in the US compared to most other countries. The Blue Mosque has someone at the entrace to check you, but for the most part they are more relaxed about it. The Hagia Sophia makes you wait in line then you go to pay a fee to get in. The fee has multiple prices and isn't clear on the difference. When you ask the person behind the window, they could care less about your question or helping you but will take your money anyway. From there you go through a metal detector and security check before finally being allowed in.
Once a church, later a mosque, and now a museum but still used as a church and looks just like a church. I guess the difference is you pay to a museum in Istanbul but not a mosque or church.
The church/Museum goes back to the Byzantium and Ottoman Empires, and is considered the epitome of Byzantine architecture.
The Hagia Sophia was the world's largest cathedral for nearly a thousand years. The current building was originally constructed between 532 and 537 by Emperor Justinian I and was the third Church to occupy the sit.
In 1453 Constantinople was conquered by the Ottoman Turks under Sultan Mehmed II, who ordered the church converted into a mosque. It remained a mosque until 1931 before being re-opened in 1935 as a museum.