Black Box Background
Subtitles contain a black background when background should be transparent.
This issue relates exclusively to TTML or STL files.
Closed or Teletext STL files can include black backgrounds. Avoid using Closed/Teletext format, instead use the Open format (sometimes referred to as DVD) EBU STL files. Using Open usually means the subtitling software defaults to using a transparent background.
TTML files can specify background color in multiple places i.e. once in the <styling> block and again in the <layout> or <region> blocks. If they are not all set to "transparent" then issues can arise. If the issue appears to be a bug in your software then contact the manufacturer. If you need to correct the issue immediately or an update/fix is not available then you can manually adjust the TTML to make all instances of "tts:backgroundColor" to equal "transparent". Be careful you do not inadvertently break the structure of the xml file or introduce any other issues.