It stands for a story filled with passion, pain and violence. The flag is also know as the Saltire - a word which also means a diagonal cross as in the Roman numeral X.
Scotland's patron saint, St Andrew, is said to have been crucified on such a cross.
A 9th century story goes that a warrior called Oengus led his northern band of Pictish and Scottish fighters into a battle with the hated Angles. The night before the fight, he vowed that if his side won he would declare St Andrew the patron saint of Scotland. The saint came to him in a dream that very night to say they would triumph. The next day, a white saltire formed of clouds magically appeared against the blue and spurred the Scots on to victory.
Blue is itself an old symbol for the Scots (remember Mel Gibson's face all daubed up with blue in Braveheart?).
Early blue flags were dyed with plant-based tints like woad and indigo. These days, rather more prosaically, the official colour for the Saltire is Pantone 300.
The referendum shines a light on what matters on our whole blue world. Look at the planet from space and there are no borders at all - we've made them all up ourselves as a way of protecting what we see as ours. At a time when borders are fading in some ways – given the power of the internet to render them pointless – the Scottish vote demonstrates that the idea of belonging to a special tribe and culture still has a firm hold on our hearts.
Whatever, putting the question to the test is a triumph for the democratic way... and a million times more appealing than taking to the field, as Oengus did, with battle screams and sharpened swords.
Some of my own favourite ancestors – so many brave pioneers – are Scottish. I'll be fascinated to see how the auld country will vote.