For the last 15 years, the lives and deeds of Washington blacks have been the subjects of museum exhibitions, books, seminars and documentaries. Now add a lively and substantive report, "Step by Step: A Story of Black Washington," which airs tonight at 9 on Channel 26.

Covering the time between World War I and the l954 Supreme Court school desegregation decision, this documentary reviews some well-known history: Marian Anderson singing at the Lincoln Memorial in l939 after the Daughters of the American Revolution banned her from Constitution Hall; President Franklin D. Roosevelt signing the executive order prohibiting discrimination in plants with federal contracts; the push to desegregate hospitals.

The documentary also shows how black Washingtonians were an integral part of these events. This is not idle civic chauvinism. As the film demonstrates, Washington, despite the restrictions of segregation, has been a hub for black leadership since the days of Frederick Douglass.

Two grand gentleman of the Washington scene provide their memories, as well as wit and analysis. They are W. Montague Cobb and Harold Lewis, both of whom taught at Howard University for more than 40 years, Cobb in anatomy and Lewis in history.

Lewis describes the Saturday night meetings of the Howard faculty at Ralph Bunche's home, where plans of action were drawn up for national and local problems. Once, the group went to the Hecht Co. to protest the segregation of the women's restrooms; on another occasion, they picketed the Justice Department because a crime conference did not include a discussion of lynching. The still photographs of this l934 demonstration, with the women in fur-trimmed coats and the men in wide-brimmed hats, wearing ropes around their necks, are some of the film's most poignant moments.

The producer, Junette Pinkney, and her associate producers, Cynthia Myers and Jeffrey Bieber, scoured the National Archives and borrowed some splendid film from the Universal Newsreel, the Harmon Collection and the libraries of the various U.S. presidents. Here we not only have recollections of how wonderful the drill teams of the segregated high schools were; we have film of cadets marching and their audiences applauding. We also see stills of whites marching toward black neighborhoods during the 1919 riots. The combination produces a spirited history lesson and a fresh, vital documentary.