E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bolt from the Blue (A).
There fell a bolt from the blue. A sudden and wholly unexpected catastrophe or event occurred, like a thunderbolt from the blue sky, or flash of lightning without warning and wholly unexpected.
Igni corusco nubila dividens,
Plerumque, per purum tonantes
Egit equos volucremque currum .
Horace: 1 Ode xxxiv. 5, etc.
On Monday, Dec. 22nd , there fell a bolt from the blue. The morning papers announced that the men were out [on strike].Nineteenth Century, February, 1891, p. 240.
In this phrase the word bolt is used in the popular sense for lightning, the Latin fulmen, the French foudre and tonnerre, in English sometimes for an aerolite. Of course, in strict scientific language, a flash of lightning is not a thunderbolt. Metaphorically, it means a sudden and wholly unexpected catastrophe, like a thunderbolt [flash of lightning] from a blue or serene sky.
German: Wie ein Blitzstrahl aus blauem Aether.
Italian: Comme un fulmine a ciel sereno.
Latin: Audiit et cli genitor de parte serena intonuit hævum. (Virgil: Æneid, ix. 630.)