A short smock-frock of a blue colour worn commonly by French workmen. Bleu is French argot for manteau.
“A garment called bliaut or bliaus, which appears to have been another name for a surcoat. ... In this bliaus we may discover the modern French blouse, a ... smock-frock.” —Planché: British Costume.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894