The Collaborative International Dictionary of English v.0.48:

Black Flags \Black Flags\
   An organization composed originally of Chinese rebels that
   had been driven into Tonkin by the suppression of the Taiping
   rebellion, but later increased by bands of pirates and
   adventurers. It took a prominent part in fighting the French
   during their hostilities with Anam, 1873-85.
   [Webster 1913 Suppl.] black fly