Janet F. Blackburn, 61, who was active in garden and service groups in Alexandria, died April 1 at Alexandria Hospital. She had a liver ailment.

Mrs. Blackburn, who was born and reared in New Jersey, graduated from the Katherine Gibbs secretarial school in New York City. During World War II, she served in the Waves, the women's branch of the Navy. She rose to the rank of leiutenant. She was stationed in Washington, where she was engaged in intelligence work, and later was assigned to the basic training of other Waves here.

After the war, she married James W. Blackburn, a media broker. The couple settled in Alexandria.

Mrs. Blackburn was a volunteer at the Alexandria Hospital and was a member of the Twig, a thrift shop that sells old clothes and similar articles and donates the proceeds to the needy. She also was active in various garden groups.

In addition to her husband, of Alexandria, survivors include four sons, James W. Jr. and Richard F., both of Alexandria, John G., of Charleston, S.C., and Edward F., of Greenville, S.C.; her parents, Mr. and Mrs. Edward L. Forbes of Fort Lauderdale, Fla.; one sister, Virginia Sullivan, also of Fort Lauderdale, and seven grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to Alexandria Hospital, or to a charity of one's choice.