WASHINGTON — Months after the Emancipation Proclamation was signed 150 years ago, the first unit of black Northern soldiers was organized as the 54th Massachusetts Regiment and went on to fight at Fort Wagner in South Carolina during the Civil War.

The group’s memory is enshrined in the 116-year-old bronze Shaw Memorial by Augustus Saint-Gaudens on the Boston Common. It was built to honor Col. Robert Gould Shaw, who led the regiment and died in battle.

Now, the National Gallery of Art in Washington has organized the exhibition “Tell it with Pride,” opening Sunday, to explore the black soldiers honored in the memorial. The gallery holds and exhibits Saint-Gaudens’ original painted plaster sculpture of the Shaw Memorial, which won a grand prize for sculpture at the 1900 Universal Exposition in Paris. It is considered among the finest examples of 19th century American sculpture....