Obscurity has nothing to do with anything. In set phrases like you're talking about, the "ao" is basically a category specifier, not a color specifier. Like aoba means young fresh leaves, not necessarily green ones, and aoyasai doesn't only mean green vegetables, etc. There are lots of set phrases like this, both obscure and common, and the color involved can be anything from white to dark gray.

But when ao is used outside of a set phrase, to tell somebody what color you're talking about, it means blue.