Rarely, does this type of mating occur and even rarer is the ability to produce viable offspring. However, we have seen this pairing intentionally conducted in a captive setting to ascertain the coloring of the offspring. The birds are called "Blutes" and are black in color with the knob like a mute swan. This coloring is extremely unattractive and most people probably would not even consider having the swans due to this unattractiveness.
The Regal Swan