An update to the development, is that I did not notice that the save format for MC 1.16 have changed slightly, so I need to adjust the parser so it can support that.
Edit: Found the fix and also sample data for black regions. In this case, it works if I draw an image, but does not if I draw a map. I am investigating and meanwhile trying to improve performance.
Edit2: The black regions are actually just regions that have been created with chunks, but no blocks have been generated. The fix for this would be to ignore them entirely. This makes it quite complicated to know what to keep and what to throw away until it has been fully processed. I may have to redesign parts of the system to make it easier to adjust toward these kind of issues.
tl;dr: A fix for the new save format has been added. The "black image output" bug has not been found.