A few years ago, when New York City was pressuring advertising agencies to hire more black executives, City Councilman Larry B. Seabrook, chairman of the Council’s Civil Rights Committee, approached one of the largest companies with a plan to address the issue. The company, the Omnicom Group, ultimately endorsed Mr. Seabrook’s plan to create a high-powered diversity committee and agreed to spend $2.25 million on initiatives. It also decided to retain a consultant from Atlanta whom Mr. Seabrook had proposed to help run the committee. The City Council speaker, Christine C. Quinn, praised the plan. H. Carl McCall, a former...