Blue on a flag usually points to one of a few things: the sky, the sea, or peace. Central America’s shared blue stripes trace back to a single 19th-century union. Eastern Europe’s blue often comes from Pan-Slavic tradition. And a wide stretch of Pacific and Caribbean nations reach for blue simply because water surrounds them.
Sky and water
Botswana · Somalia · Fiji · Tuvalu · Kiribati · Marshall Islands · Micronesia · Palau · The Bahamas · Israel
Central American Union heritage
Guatemala · Honduras · El Salvador · Nicaragua · Costa Rica (blue stripes carried over from the same 1820s federation, even though Costa Rica later added red)
Pan-Slavic colors
Russia · Slovakia · Slovenia · Serbia · Croatia
French and European tricolor tradition
France · Luxembourg · Chad · Romania · Moldova · Andorra · Iceland · Estonia
Peace, unity, and post-independence redesigns
Rwanda (blue added specifically to represent peace and hope after the 1994 genocide) · Guinea-Bissau · South Sudan · Somalia (also drawing directly from the UN flag’s blue, given Somalia’s flag was designed under UN trusteeship)
Everywhere else blue shows up prominently
Argentina · Uruguay · Greece · Finland · Sweden · United Kingdom · United States · Nepal · Panama · Venezuela · Ecuador · Colombia · Kazakhstan · Cape Verde · Botswana