Allrighty then, I read the article Reijo mentioned (in Helsingin Sanomat / Kuukausiliite, october 2008)
The article is not a joke, not a wild guess but six pages long serious investigation "who is the person behind Bianca Castafiore". Farr's writings and speculations are well known but could there be someone else, a real operasinger, behind Bianca? Maria Callas (1923-77) and Renata Tebaldia (1922-2004) were too young to be Bianca, Belgian Clara Clairbert (1899-1970) was one possible canditate but she never sang Marghareta's role in Faust.
Finnish Aino Ackté (born 1876) could be the diva behind Bianca. She won operasingingcompetition in Paris by singing Faust. She made her debut in Paris Grand Opera (1897) in the role of Marhgareta.
One peculiar thing happened when Ackte sang New York Metropolitan(1904-06): the name of the official operaphotographer in Metropolitan was Dupont, he was Belgian!
Aino Ackte has recorded the jewel song many times. Listen to this:
http://yle.fi/elavaarkisto/?s=s&g=8&ag=47&t=325&a=2485You can compare it to Bianca's performance in Seven crystal balls, put the focus on how she introduces herself.
And there more connections, Ackte's sister's name is Irma and Richard Wagner can be assosiated in many ways to Aino Ackte's life and carrier (not very interesting)
But there is still one very strong bind between Cartafiore and Ackté. Ackte is a founder of Savonlinna opera festifal. The festifal is arranged in a castle called Olavinlinna (google "olavinlinna"). In Herge's archives is a picture of Olavinlinna and in black and white version of King Ottokar's Sceptre there is exactly the same castle, its name is Kropow of course. In colourversion (after the WW II ) one tower more has appeared. That tower is from Finnish Vyborg castle. Maybe it is a symbol of symphaty for Finland. Kropow is backwards almost Vyborg. Weird.
And there is lot more. You could try to find Philippe Goggin's article in "Tintin a la decouverte des grandes civilisations. Philippe has invented the connection between Kropow and Finnish castles.
I would like to show you the picture of the Herge's phothograph of Olavinlinna and the original black and white frame of Kropow, but I can't, I don't know how to add a picture to this message.
EDIT (19.10)
The mainpoint is: If the source of inspiration is a real opera prima donna, Aino Ackté is the best canditate. Herge must have heard Ackte's version of Marghareta or even he might have seen her singing the jewel song in theatre. And the other connections, Dupont, Irma ... Vamos a ver?
I am so glad that Bianca Castafiore might be a Finnish operasinger and our castles might play a little role in Tintin. :-)
EDIT (20.10)
I meant Igor Wagner can be connected to Ackte's carrier in so many ways.