A "widely known fact"?  I've been a major fan for over 25 years and it's the first time I've ever heard this "fact".  It's certainly never appeared in any of the dozens of biographies written about her, and her brother, Christopher, didn't mention it in his book about their life together.  In Europe (and especially in Italy), it is often asserted that her real name is "Louise" and that "Madonna" is just a stage name, but it is a "widely known fact" that she was named after her mother, Madonna Fortin Ciccone.