Based on a historical event, this song expresses the idea that things are not always as they seem and that absolute truth may not be the truth at all!
Until the mid 17th century Europeans believed so strongly that 'All swans are white' that this became a saying used to prove that something was beyond doubt.
When Willem de Vlamingh sailed to New Holland (Australia) in 1696 he discovered a river on the south-west coast of the continent where the swans were...black.