Back then blacks were slaves in the South and had no choice but to fight with their owners. If their owners told them to fight for the Confederacy do you really think saying no would be an option to a slave?
Your statement is false.
Blacks who fought for the South were given their freedom in return for them volunteering.
None of them fought as slaves, and none of them were forced to fight.
It may be hard for some people to grasp, but it was THEIR culture, too. And there were lots of black slave OWNERS.