E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 ufs de mouches sur and a fly-blow is un uf de mouche. The word seems to be connected with blot, the egg of a moth or other insect.