For whatever reason, the agreed upon surrender signal for U-boats was a black flag. Don’t know why.
There actually was a book about the U-Boat surrender aptly named: Black Flag: The Surrender of Germany’s U-Boat Forces Hardcover – October 29, 2009, by Lawrence Paterson (Author)
Other references to using a black flag.
The Anarchist black flag has been an anarchist symbol since the 1880s. Anarchists use either a plain black flag or a black flag with an “A” and an “O” around it, this symbol is a reference to a Proudhon quote, meaning anarchy is the mother of order[citation needed] .
The color black was famous as the flag of Italy’s National Fascist Party, designed after the party’s paramilitary Blackshirts.
The color was also chosen by leftist Nazi activist Otto Strasser in his creation of the counter-Nazi party; the Black Front.
Upon the surrender of Nazi Germany in World War II German U-boats were ordered to fly a black flag and sail to an Allied port and surrender.
SPECIAL ORDERS BY THE SUPREME COMMANDER, ALLIED EXPEDITIONARY FORCE TO THE GERMAN HIGH COMMAND RELATING TO NAVAL FORCES ordered U-Boats to Fly a large black or blue flag by day.
Apparently the Japanse did the same thing
Black Surrender FlagI-400 flying the black flag of surrender
http://www.pbs.org/wnet/secrets/japanese-supersub-image-gallery/552/