(an oath). You be blowed (an oath), a play on the word Dash me, which is a euphemism for a more offensive oath.
“ `Well, if you won't stand a pint,' quoth the tall man, `I will, that's all, and blow temperance.' ” —Kingsley: Alton Locke, chap. ii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894