Is it permissible to use the word "Sudanese" to describe something black as a metaphor for blackness, and is this considered prohibiting what Allah has made lawful or mocking?

There is no harm in describing a black object as "Sudanese," unless the intention is to mock or ridicule people with black skin. Your statement, "This is unlawful," in this context is not a prohibition of what Allah has made lawful, because the assumption that the prohibition stems from mockery is correct. It seems the questioner is suffering from obsessive doubts regarding matters of lawful and unlawful, and he should turn away from these delusions and thoughts, and not allow Satan a path to him.