On 13 July 1878, the great powers of Europe signed the Treaty of Berlin, recognizing the independence of Montenegro, Romania, and Serbia from the Ottoman Empire.
At the time, Montenegrins generally considered themselves part of the Serbian nation. Their first constitution even said “there is no other nationality in this land except Serb nationality”. So in 1860 Prince Danilo adopted the Serbian tricolour as his country’s national flag. The one difference was the shade of blue.
Honestly I’m not sure what prompted the change. Could have been a desire to differentiate the country a bit, or it could have something as simple as the available materials when the flag was first designed.
Montenegro (naval jack, 2010-Present)
Montenegro adopted a distinct naval jack in 2010, essentially a square version of its national flag with a blue background. They didn’t really need a distinct naval jack since their flag is already plenty unique, but what the hell, I like it anyway. It’s a smart looking flag with an attractive colour scheme.
The little square bump near the top of this flag is an all-purpose symbol representing all the major monuments and landmarks in the city. Now that’s efficiency.
Serbia and Montenegro (proposed, 2004)
During its short time on this earth, the State Union of Serbia and Montenegro flew the flag of its predecessor, the Federal Republic of Yugoslavia. But for a while there was talk of replacing the old flag.
At the time, Serbia and Montenegro had nearly identical tricolours (the only difference was that Montenegro’s was longer and had a much lighter shade of blue) so most proposals centred around combining them somehow. The leading proposal in 2003 was a tricolour which used the average shade of blue between the two flags. In June 2004 a four-coloured flag with one half-stripe of Serbian blue and one half-stripe of Montenegrin blue was proposed.
I don’t know why this flag was never approved, but I bet it has something to do with Montenegro adopting a completely different flag a few months later. The union was dissolved after Montenegro declared independence on 3 June 2006.