They should do just fine. The only problem may be during mating season as the male may attempt to mate with the female. The probability of them actually mating or producing a clutch being from two different species, is rather low, but stranger things have happened.

The black swans tend to be a little more aggressive. You can alleviate some of these problems by placing both birds in a nearby pen and letting them get accustomed to each other prior to releasing on the pond. If the pens are not available and you cannot top them to protect from predators, then standby with a boat to observe any extremely aggressive behavior so that one swan does not get severely injured or killed. If any aggression is seen, then you probably need to rethink placing both together. As long as the mute (white) swan is not overly territorial, this match should be okay. The Regal Swan