"like another instance of great fortune much later in my life"

This 'other instance of great fortune' which has its roots in slavery is, if we take Stingo to be Styron's double, probably the success of Styron's 1967 Pulitzer Prize winning novel, The Confessions of Nat Turner, based on the life of the slave rebellion leader of that name. For more information see the bookmark to p. 516: "Nat Turner, I said, was a Negro slave"