in the phrase “fly-blown,” has nothing to do with the verb to blow (as the wind blows). It means that flies have deposited their eggs and tainted the article. In French, deposer des oeufs de mouches sur ... and a fly-blow is un oeuf de mouche. The word seems to be connected with blot, the egg of a moth or other insect.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894