When the Angles and the Saxons conquered what is now England, they (and thus the new upper crust) were as a group much blonder than the preexisting population. Thus was born the equation Fair = nobility = beautiful and of course brown/dark = lower or working class = unlovely.
Mama tells sonny: "Therefore I charge you, on my blessing, go bring the brown girl home!" This is not a request; this is not advice. This is an order. If sonny expects to inherit rather than be disowned, he'd better bring the brown girl (and the bacon, as it were) home.
This text is from the great Folkways set of LPs featuring A.L. Lloyd and Ewan McColl, back in the 50s, titled The English and Scottish Popular Ballds. Alas, Smithsonian-Folkways today only has the McColl half of this set, for some reason unknown to me.