Black Flags,
an organization of Chinese rebels who established themselves in the Red River valley in Tonquin, after the suppression of the Taiping Rebellion in Southern China (1850-1854). From their warlike character and desperate deeds they were called the Black Flags as distinguished from the peaceable Yellow Flags. They were responsible for the massacre in 1884 of missionaries and native Christians.
Entry from Everybody's Cyclopedia, 1912.