did you search this forum and check for example the export options ?
you should figure out, wether it is an export problem or an import problem.
Did you have any .dxf from the target Application that allows black ?
if you want to make sure you need to look at the generated .dxf ( recommend a simple one, with one single line) - it is a readable text-file
and the specifications can be found online:
→ check if Rhino writes a proper black
→ if yes it is an import problem and you have to see the Target Application ressources.
→ if no it is an export / Rhino-Problem and you can report a bug here.
And to have others to profit from your problem: please post the results here - including the name of your “target-application” … something like "export dxf from Rhino - import in “my target application” …
hope that helps -best -tom