At one end of the square, there is an innocent-looking 19th-century red- brick townhouse with shuttered windows and wrought-iron balconies. Every hour, without warning, there emerge from the windows - breaking the balconies, roaring and groaning horribly - six enormous, golden dragons' heads, feet and a tail.
It is a pleasant joke. The house has been converted at huge expense into the world's first museum devoted entirely to magic (also known more pompously as the International Conservatory of Magic and Illusionary Arts). The building is itself a magic trick that never ceases to amuse.
According to some critics, the Maison de la Magie, which opened this summer, has performed another one-off trick: making pounds 7.5m in public money disappear for no particularly good purpose. The idea of having an institution and place of entertainment devoted entirely to magic as an "art" was conceived eight years ago by Jack Lang, the minister for culture in the then Socialist government. Mr Lang was - and is - also the mayor of Blois.
His adopted town by the Loire was the perfect site, he said, because it was the birth-place of the man who "invented" modern magic: introducing the first scientific use of mechanical, illusionary devices and also the tradition of magicians' wearing evening dress. His name is not now celebrated much outside France, except at one remove.
He was called Jean-Eugene Robert-Houdin (1805-1871); he was internationally famous in his day and, among other things, a friend and fellow ghost- and fake-medium-buster of Dickens. After his death his name was stolen, and his magical achievements denigrated, by the American illusionist and escapologist, Harry Houdini (born Erich Weiss).
He remains, however, a revered name among professional magicians all over the world. The American magician, David Copperfield, made a pilgrimage to Blois recently. The Dutch "punk" magician, Philippart, who is performing in the theatre, says: "The opening of this place is the second-biggest talking point among professional magicians around the world this summer."
Only the second? "That has been taken by the creep who is revealing illusionists' secrets on an American TV series."
The Maison de la Magie takes no such liberties with the code of secrecy of the profession it celebrates. In the attic of the building is a priceless collection of illusionists' tricks down the ages. That floor of the building is strictly reserved for professional magicians.
This dual function - museum/entertainment centre and "conservatory" of magical arts - is the source of some of the criticism of Mr Lang's bright idea. His original conception was much more cultural and less commercial: that trick failed to come off, partly because of a change of government; partly because of a shortage of money. The concession on the public and live performance part of the building has now been let to the company which runs the Parc Asterix north of Paris.
Is an entertainment centre a proper use of public funds? Mr Lang is dismissive. "You could not have an institution devoted to magic without some element of entertainment. But the intention also is to make a bridge between magic as an entertainment and as an art, which helped to advance scientific knowledge."
Robert-Houdin's tricks, Mr Lang said, used electricity long before it was explored commercially. His experiments helped to prepare the way for the discovery of cinema (the greatest magical trick of the 19th century).
It is up to the French taxpayer to decide if the Maison de la Magie is in another example of losing a sense of perspective. The delight of visitors, especially the young, suggests that the trick has just about worked.