Before Europeans discovered Australia, Europeans had no reason to believe swans could be any colour other than white, and on seeing black swans they revised their beliefs.

Judge on the historical record. True: before Europeans encountered Australian black swans (Dutch voyages, notably de Vlamingh in 1697), Europeans had no credible observations or reports of non-white swans and treated swans as white (e.g. 'rara avis... nigroque simillima cygno' used as a figure for the impossible), and after the sightings they accepted that black swans exist. Mostly true: this broadly holds but with a minor slip, e.g. Australia was 'discovered' by Europeans (1606) some decades before black swans were reported, or there were a few scattered earlier hints. Mostly false: there were known European or earlier reports of black swans that were widely available before the Australian sightings, so the claim overstates. False: Europeans already knew of black or non-white swans before reaching Australia and generally accepted that they existed.

Set before the research began.

1. E1 Symbols of Western Australia Government of Western Australia (Department of the Premier and Cabinet) · 27 April 2020

   “However, it was not until the voyages of Dutch explorers to the Great South Land in the 17th century that the existence of Black Swans was reported to the rest of the world. Until then, people in Europe believed that all swans were white.”

2. E2 Friday essay: a rare bird — how Europeans got the black swan so wrong The Conversation (article by David Haworth, Monash University) · 10 June 2021

   “In 1636, Dutch mariner Antonie Caen observed black birds “as large as swans” at sea off Bernier Island — probably the first recorded European sighting.”

3. E3 Friday essay: a rare bird — how Europeans got the black swan so wrong The Conversation (article by David Haworth, Monash University) · 10 June 2021

   “The expressions made sense because Europeans assumed, based on their observations, that all swans were white.”

4. E4 Friday essay: a rare bird — how Europeans got the black swan so wrong The Conversation (article by David Haworth, Monash University) · 10 June 2021

   “Reports of the black swan made it back to the Netherlands and then to England, but it took another century for its mythical status to dissipate completely.”

A Western Australian government page (E1) says black swans were not reported to the rest of the world until the Dutch voyages of the 17th century, and that until then people in Europe believed all swans were white. An independent essay (E3) says the old European 'black swan' expressions made sense because Europeans assumed, from what they had seen, that all swans were white. It describes a 1636 sighting of black birds 'as large as swans' as probably the first recorded European sighting (E2), so the sightings came during the Dutch voyages rather than at the very moment of 'discovery'. The same essay says it took another century for the black swan's mythical status to fade completely (E4), so Europeans did revise their beliefs, but slowly. That is a minor qualification to the claim.