Just to clarify, the Blue Mosque in Istanbul (properly speaking, Sultan Ahmet Mosque) was never a Christian church. Its neighbor in Sultanahmet Square, the Hagia Sophia, however, was a church. It was converted to a mosque by Mehmet II when he conquered the city. (Mehmet is also known as 'The Conqueror' or 'al-Fatih' in Ottoman Turkish meaning the same, and gives this name to the Fatih district of the city where both of these are found.)

It remained a mosque (and was extenisvely renovated as such) until the abolition of the Ottoman Caliphate and establishment of the Turkish Republic when Ataturk had it turned into a museum in 1935.

Whether or not it should be restored as a mosque is a somewhat active political issue in Turkey to this day.

As for other conversions of churches into mosques, it depends on circumstance.