Hi, Rory!
Thank you for your question: Yes, there were Australian Black Swans in Stanley Park, from the late 1970s to early 1980s. In 1977 one of them had two cygnets. They were moved to another area of the park, and in the end, the swans were stolen.

Here are two archival references that might be of interest. Unfortunately, there's not much more documentation but I hope this helps.

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Reference No. 1
"New addition to Stanley Park The newest family in Vancouver's Stanley Park are a pair of three-day-old baby Australian Black swans and their mother. After the eggs hatched in a nest in the west end of Lost Lagoon, the Black Swans had to be moved when they were battled by white swans over territorial rights. The Black Swans are now in the duck pond next to the flamingos."
Winnipeg Free Press: Tuesday, April 12, 1977 – Page 54

Reference No. 2
"Mike MacIntosh, of the Vancouver Parks Department, who has been involved with the swans in Stanley Park since the 1960s, is not concerned with their low birth rate. “The nine we have now is a much more realistic and manageable number.” In the past they were much larger in number in the park, and were kept company by a few Australian Black Swans that were subsequently stolen, and some native North American Trumpeters and Tundra Swans, that eventually moved on."
http://carlamarialucchetta.com/tag/lost-lagoon/