Ethiopia is a coastal nation and long had contact with the Arab and other ancient civilizations. I think it's also mentioned in the Bible.
I'm not sure the Nubian example is accepted as a nation's use of a written language. A few years back Henry Louis Gates had a history series on PBS, and one of his episodes addressed "evidence of a written language" used by the Nubians. If it were an accepted fact that written languages had been developed by black Africans, Gates would not have been sort of suggesting that it was a possibility. It seemed there was a very limited use of writing, but not a well developed written language.
And what’s the big deal about contact with ancient Egypt? Its not like our written language doesn’t eventually go all the way back to the Phonecians.
Maybe that is the big deal. The use of written language spread far and wide in some directions, but not in others.
Personally, I think we (European-descend civilizations) had the advantages of Greece and Rome and their Phonecian-descended alphabets. They created a tradition of writing and literacy unlike anything in the rest of the world.
In Greece and Rome, every citizen, rich or poor, was expected to have some degree of literacy. Other countries developed forms of writing but literacy was restricted to elite groups like scribes or Mandarins.
If my language required 3000 characters for basic literacy, I’d be illiterate, too.
Of course, that says something scary about modern Chinese...