By the time he appeared on the public stage in 1990, he was using the name Bill de Blasio as he explained he had been called "Bill" or "Billy" in his personal life.[10] He did not legally change over to this new name until 2002, when the discrepancy was noted during an election
http://en.wikipedia.org/wiki/Bill_de_Blasio#New_York_City_Council_.282001.E2.80.932009.29
There are only two reasons why I'd ever have my name changed.One is if my legal birth name was something Poophead Bumbleeper.The other is if I had something in my past that I wanted to hide.