Blackwell was born in Whitsett, Fayette County, Pennsylvania on August 1, 1931. After attending public schools, Blackwell joined the United States Army, serving in the Korean War in 1953. Blackwell was a boxing champion during his years in the Army.

Blackwell was a powerful Philadelphia union leader, serving as president of the Local 1332, International Longshoremen’s Association from 1973-1991. Blackwell's professional political career began with election to the Pennsylvania state house of representatives where he served from 1973-1975. However, Blackwell was best known as a vibrant member of the Philadelphia City Council from 1974-1991. In City Council, Blackwell served for several terms as Chairman of the City Council Finance Committee, where he led the charge to divest pension funds from businesses doing business in South Africa. Blackwell also sponsored the Philadelphia's first law to create opportunities for minorities and women to compete to obtain city contracts. Blackwell was also heavily involved in legislation to create the Pennsylvania Convention Center and in passing the law that broke Philadelphia's long-standing building height limit, allowing for the construction of Philadelphia's One Liberty Place. Blackwell was perhaps best known for his fiery oratory on the Council floor and for serving as a mentor to the current Philadelphia Mayor (and former Council President) John F. Street. During his City Council tenure, Blackwell was an unsuccessful candidate for mayor of Philadelphia in both 1979 and 1991.

Blackwell was elected as a Democrat to the One Hundred Second Congress by special election to fill the vacancy caused by the resignation of Representative William H. Gray, and reelected to the succeeding Congress (November 5, 1991-January 3, 1995). In Congress, Blackwell was a member of the U.S. House Committee on the Budget and a reliable advocate for President Bill Clinton's economic policies. Blackwell was ultimately an unsuccessful candidate for renomination to the One Hundred Fourth Congress in 1994, losing the primary to Chaka Fattah, and served as lobbyist following his tenure in Congress.

On January 24, 2003, Blackwell died at the age of 71. A mural reading "Thank you, Mr. Blackwell,” can be seen at 42nd Street and Haverford Avenue in West Philadelphia. Blackwell's widow, Jannie L. Blackwell, is a member of the Philadelphia City Council.