I've generally taken it to mean, in its basic meaning, the particular menu item that the management of a luncheonette has chosen to be the one they are featuring on a particular day. Usually it implies a special (lower than usual) price or it may be an item that is not served every day. Why the 'blue plate' I couldn't say. It would take on a metaphorical meaning of something a little out of the ordinary. I think it's pretty wide-spread in USn usage.