Yes, he was a former slave himself but he became successful and fought.
I don't see that he was a former slave but his mother was, which would make him eligible to be one, too. Interesting story, that. Also, that of Randall Gibbons, who was also a 'black Confederate'. His great-grandfather was a free man of color - same as our Mr Buckner - but General Gibbons was white for all he knew...or would admit.
This is who your black Confederates really were, General. They were people who owned slaves themselves, or people who were descended from free blacks who had owned property - General Randall Gibbons of Louisiana. Another black Confederate was General Jefferson, grandson of Thomas Jefferson, who was by Virginia law considered colored and happened to be recognized by another slave from Monticello...General Jefferson was passing for white and was married to a white lady, which would get him hung and his children sold off if it was known he was 'one drop'. They were people who were invested in slavery, or in keeping their true heritage a secret.
People who were brought to the war by their masters might well have been shooting right along with the master - he, too, had an investment in the plantation as that's where his family was and they were under the same master's control. So you will find many official reports, letters and other secondary source information attesting to this. You'll also find they were mainly employed as menials and never considered soldiers.
Forrest's 47 men who served with him in the war are in this category. One may find several Union reports of battles noting blacks among Forrest's troops but you will never find one enlistment paper or pension application for any of them as soldiers.