Slaves were generally prohibited from being taught to read and write. Once emancipated the need would arise for them to sign documents, and like illiterate whites, they would be asked to, "Make their mark," which would generally be a simple, "X".
Thanks for the reply and a valid explanation as to why some blacks change their name. However, it is done by Black Muslims to a great extent, MALCOLM X etc. Dallas shooter Johnson has the middle initial X (xavier) but I think there is a tie between the New Black Panthers and Black Muslim Organizations. Cannot trust the FBI or Admin to make a truthful statement. If it were a truthful statement to the negative, it would still be disbelieved by many. JMO