The slaves that fought for the Union were promised freedom, and land, if they fought for the Union, so they gladly volunteered. Yes, they were in segregated units, and yes, they were the tip of the spear in many cases. But the reason why they were glad to fight was because they were fighting their oppressors. This was their Revolutionary War.
Your last paragraph is rather insulting to the lives of Black slaves. They may have been on the plantation, but unless you were a house slave, you lived in complete squallor.

