alleged early modern Danish flag of mourning (Sorgeflag). 
according to one Jan Oskar Engene, posting 7 March 1996 on flags of the world:

"Until 1743 a flag with proportions identical to the Dannebrog but with black instead of red was used a sign of mourning. In 1743 it was decided to use the Dannebrog flown at half mast instead." No reference was provided. It is also a black and white variant too.
own work, based on Image:Danish flag of mourning.gif
Jan Oskar Engene; Andreas 06