There were many songs written about blackbirds and bluebirds during the 1920s and '30s. Why was this such a popular topic? While there has been much speculation about this particular song there doesn't seem to be much discussion about the repeated use of this metaphor. Here, I think, the simplest explanation speaks the loudest: blackbirds represented bad luck and bluebirds brought good luck. During the depression everyone was hoping that the blackbirds would fly away and bluebirds would take their place, bringing sunshine and restoring hope.