He characterized the American Negro Slave as unique of all the black slaves of the world because his identity was impossible to trace unlike the other slaves like the Haitians who could trace their origin to some African kings.
Explanation:
James Baldwin in Paragraph 17 of his essay, 'Stranger in the Village', mentioned that the American Negro Slave was unique of all the black slaves in the world because while the other Slaves could still trace their identities down to somewhere, the American Negro Slave could only trace his identity to the bill of sale which was the entrance paper of his forebears into the American society.
Some Haitians could trace their origin to some African kings but that was quite difficult for an American Negro slave to do. Thus, came his search for his identity within the American society.