Black Execution Flag

Today I watched the excellent movie "The Verdict" from 1946 (German title: Hier irrte Scotland Yard; starring Sidney Greenstreet, Peter Lorre). The story plays in London of 1890; until the very end you can't be sure about the murderer. At the beginning a black flag is hoisted over the prison to indicate to the public that the convict has been executed (in the movie an innocent victim of an intrigue). This raised the question to me what tradition those black flags have? Does anybody know more about it? In which countries and in which periods did black flags indicate an execution?
Martin Karner, 21 November 2004

Another example is Thomas Hardy's novel 'Tess of the d'Urbervilles. A Pure Woman' (1891), where right at the end you have the same scene. Now Hardy's world ('Wessex') is imaginary but this may have been real. British list members will know more on this subject I suppose.
Jan Mertens, 21 November 2004

I have never heard of this practice in the United Kingdom, although of course executions did used to be carried out in public. Would like to know if anyone else has information on this.
Colin Dobson, 22 November 2004