The "blue collar" bit refers to Zhou dynasty scholar robes. A bit weird that Legge is translating it to blue though -- the color should be black. (The word used now means blue/green usually, so it seems a mistranslation.)

Baidu says historically scholars thought it dealt with a school that was disbanded and the scholars dispersed. And/or a poem about depravity (? I'm not quite sure of this). But modern scholars think this is a love song.