Segregation by law, or de jure segregation, was the practice of separating people on the basis of race or ethnicity.
"The armed services weren't desegregated until 1948," Howard said.
President Truman issued an executive order ending racial segregation in the United States military forces, Howard added.
Even so, Howard said the black soldiers all received honorable discharges and shouldn't be forgotten.
"These soldiers lived in a time when things were pretty rough for African-Americans," he said.
The National Park Service currently is doing research to find out more about the soldiers, who Howard said have never received visits or inquiries from family members or friends during his 10 years at the cemetery.
"We're not even sure exactly how they ended up here," Howard said. "We know what divisions they served in, but we want to find out more about them."
Antietam National Cemetery was established in 1864 as a burial place for Union and Confederate soldiers killed in the Battle of Antietam, which took place in the fields surrounding Sharpsburg and was the bloodiest single-day battle in U.S. history.
"There were 23,110 causalities killed and wounded," Howard said. "That's about one soldier for every second that the fighting went on."
Following the Battle of Antietam, which took place on Sept. 17, 1862, President Lincoln issued the Emancipation Proclamation, which freed slaves in the South, Howard said.
The segregated burial sites for black World War I and World War II soldiers represented a nation that still was deeply divided along racial lines at the time, cemetery historian Ted Alexander said.
"This country was still grappling with these issues long after fighting a bloody Civil War," Alexander said. "These soldiers were among troops of all races to fight in World War I, the war meant to save Democracy and end all wars."
It's a story that now is told by cemetery park rangers during walking tours that attract more than 250,000 people annually to Antietam, Howard said.
"This separation, even in death, is the greatest irony that we can show people - the fact that this was occurring some years after the war ended here at the battle that was to free all men," Howard said.
Today, the cemetery is the resting place for more than 5,000 soldiers killed in the Civil War, the Spanish-American War, the Korean War, World War I, World War II and one soldier killed in the 2000 terrorist attack on the USS Cole, Alexander said.