It is if she is instructing it to blow southerly in the adverbial sense ~~ i.e. from the south ~~ see above clear definition. She is saying "blow the wind from the south", with 'blow' in the subjunctive: i.e. the line means "Let the wind blow from the south".

Get it?

Re 'barque', btw, the first definition in Chambers Dictionary is the tech one of a square-rigged fore-&-aft vessel; but the 2nd definition is "any boat or sailing-ship (poetic)", which I have always taken to be the usage here.

~M~