Bryce, in the Photgraphic History of the Civil War, originally published in 1911, there is a posed picture of to black soldiers taking cover behind a house which has been hit by a solid shot. The caption indicates they were defending against Early's attack on Washington.

http://www.nps.gov/history/history/online_books/civilwar/hrs1-7.htm

The above site has a narrative of Early's attack, listing Federal units. USCT units are not mentioned, but the story indicates there were scattered small groups reponding. Stan