The Black Native Americans were not illiterate. Many of these Blacks continued to use the Vai script to write. The Vai script taken to America by the the Malian explorers led by Abubakari in 1310. They left inscriptions across the United States.
There is constant talk about the Cherokee script.
Although a great deal is made of the invention of the script, in reality it is just a modification of the Vai writing that had been used as a written language in America at least since 1310. You can tell the influence of the Malians on Sequoyah or George Guess. You can look at the African influence on Sequoyah by his head dress which is a Turban.