As slaves they didn't have much of a say in the matter. But in any case virtually all served in supporting roles and not as combat troops.
It has been estimated that over 65,000 free blacks where in the Confederate Ranks. Over 13,000 of these, "saw the elepant" also known as meeting the enemy in Combat
Dr. Leonard Haynes a African-American history professor at Souththern University has stated, When you elminate the Black Confederate Solider you've elminated the History of the South.