Benjamin K. Blackburn, 56, an administrative law judge for the National Labor Relations Board, died Saturday at his home in Baltimore following a heart attack. He was stricken while mowing the lawn.

Judge Blackburn, who was born in Baltimore, joined the NLRB in 1958 as an attorney. He became a trial specialist and supervisory attorney and served in the agency's Washington headquarters and in its Baltimore and Chicago regional offices.

He was appointed an administrative law judge in 1963, conducting administrative trials of cases in which unfair labor practices have been charged. He was assigned to the Baltimore regional office and remained there until his death.

Judge Blackburn served as an instructor in the field artillery in the Army during World War II. He earned a bachelor's degree from the University of Chicago in 1947 and then returned to Baltimore. He earned a law degree from the Unversity of Maryland in 1951. He was an editor and an editorial writer on The Baltimore News-Post, now The Baltimore News-American, in the early 1950s. He conducted a private law practice briefly before joining the NLRB.

Survivors include his wife, Ann, of the home, two sons, David, of Baltimore, and Brian, of the home, and a daughter, Robin, also of the home.