You are Hans' sister, are you not? Hans and I have been shipmates aboard SS JOHN W BROWN, although I haven't seen him in several years. You and I have conversed before and I think I remember you sending me a copy of a book you had written.
Anyway, I came across a reference to the use of a black flag in convoy operations and remembered your long-ago inquiry.
According to my source, "[t]he black flag was flown when an attack was imminent or in progress."
See http://www.stexboat.com/books/Ethnography_MarineConvoys_WWII.pdf, page 120.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website