He used to be Warren Wilhelm Jr. From Wikipedia
He eventually adopted his mother’s family name of de Blasio because his father was “largely absent,” and he wanted to embrace his Italian heritage. In 1983, he changed his legal name to Warren de Blasio-Wilhelm, which he described in April 2012: “I started by putting the name into my diploma, and then I hyphenated it legally when I finished NYU, and then, more and more, I realized that was the right identity.” By the time he appeared on the public stage in 1990, he was using the name Bill de Blasio, as he is called “Bill” or “Billy” in his personal life. He petitioned to officially change his name to Bill de Blasio in December 2001, after the discrepancy was noted during an election.
[Comment at Unz.com