On the face of it, this might seem like a fairly obvious expression to trace. The “bolt” is of course a bolt of lightning, and the “blue” is the sky – the idea being that when the sky is blue and clear we don’t generally expect bolts of lightning!
The idiom has been linked with ancient mythology, where an angry god, such as Zeus or Jupiter, could be imagined hurling bolts of lightning from a clear sky to show his displeasure. Indeed, this idea has carried on today, with people joking that they may be “struck down” if they enter a church, and so on.
Let’s take a look at the earliest known example in writing:
cogor relictos: namque Diespiter
igni corusco nubila diuidens
plerumque, per purum tonantis
egit equos uolucremque currum
For Jupiter, Father of all the gods,
Who splits the clouds with his lightning,
Flashing away, drove thundering horses,
And his swift chariot, through the clear sky
This comes from Roman lyric poet Horace (Quintus Horatius Flaccus), in his Ode XXXIV dating to around 23 BC.
The first translation into English of Horace’s Odes was in 1566 by Thomas Drant. As people became more and more familiar with the imagery, the idiom spread.
It first appears in its current form in English in The French Revolution by Thomas Carlyle from 1837, where he says:
“…sudden really as a bolt out of the Blue…”
Such casual use of the phrase suggests that this form was already in common use and generally well known, notwithstanding the fact that Carlyle was a well educated man who would have been familiar with Horace’s works.
The same idea is expressed in Swedish, with blixt från klar himmel, which translates as “lightning bolt from a clear sky”. This also extends to its geographic neighbouring language, albeit from a different language family, Finnish, in which you say salama kirkkaalta taivaalta, also meaning “lightning bolt from the clear sky”. (Here, kirkas taivas means “clear sky”, and this is an interesting example of how Finnish words use suffixes to build up meaning in one word, while these things are expressed with prepositions and other words in English.)
Over to Eastern Europe, Bulgarian uses a very similar expression, with Като гръм от ясно небе [kato gram ot iasno nebe], meaning “like thunder from a clear sky”. Fellow Slavic language Slovenian uses the same idea, with kot strela z jasnega, which also translates as “like a thunderbolt from a clear sky”, again representing something sudden or unexpected.
Hungarian is another one to use the same kind of expression, with derült égből villámcsapás, meaning “a thunderbolt from the clear sky”, while German also thinks along similar lines, with aus heiterem Himmel, which literally means “out of the clear sky”. Far away geographically, but close in thought comes Manx with haink eh myr taarnagh veih’n speyr; “thunder from the sky”.
We come now to Portuguese, which uses the idiom cair do céu, which means “falling from the sky”, which makes you think that something was so unexpected, it was like it had fallen from the sky. After all, things don’t generally fall from the sky!
There is a very interesting expression in Welsh used for something unexpected; fel huddyg i botes, which translates as “like soot into soup”. Evidently, this idiom dates from the days when everyone cooked over a real fire and soot may have fallen from the chimney into a bubbling pot. Indeed, especially with a careful cook, soot in soup is neither expected nor desired!
Personally, I rather like the equivalent expression in Japanese, which is nemimi ni mizu, meaning “cold water on a sleeping ear”. This brings to mind an amusing picture of a bucket of cold water being poured over the sleeper to wake them up. What a cheerful way to start the day!