PHILADELPHIA — It’s a once-in-a-generation artifact that rings strikingly relevant: a Civil War battle flag carried by a Philadelphia regiment of Black soldiers — and hand-painted by David Bustill Bowser, the son of a fugitive slave and Philadelphia’s most acclaimed 19th-century Black artist. On deep-blue silk with gold fringe, the regal regimental standard of the 127th United States Colored Infantry regiment depicts a soldier off to war, bidding adieu to Lady Columbia, the goddess of Liberty. An inscription captures the bitter burden set upon Black soldiers fighting for freedom: “We will prov…