Was the most recent common ancestor of all swans black, or white, or some other color? How do we know, and how certain can we be?
According to the known phylogeny of Anserinae in this paper,
Cygnus atratus (the black swan) is most closely related to Cygnus melanocoryphus (the black-necked swan). All of the other swans in the genus Cygnus are white, or close to white.
Therefore, it is reasonable to assume via maximum parsimony and maximum likelihood that the two genera evolved their blackness from a white ancestor, and not that all of the swans independently turned white from a black ancestor.