He was called "Du" because he did.
-----------------------
http://www.baltimoresun.com/features...,7019554.story
Clarence H. Du Burns
First black mayor of Baltimore
1918 - 2003
Clarence H. Du Burns, a self-made politician who rose through grass-roots involvement in his native East Baltimore to become the city's first African-American mayor.
Throughout his career, Mr. Burns never let the public forget his humble beginnings, growing up in a house at 424 N. Caroline St., his jobs hawking newspapers and vegetables, and getting hired as a locker room attendant at Dunbar High School the old-fashioned way - through City Hall connections. In the span of just 15 years, with political acumen, charm and personal warmth, he ascended from that modest job to the city's highest office.
He got the nickname "Du" in the 1940s when he was knee-deep in local politics - always doing things for people. Among his many deeds was delivering votes for Mayor Thomas D'Alesandro Jr., the task that helped him land the locker room job.