A great many Blacks loyally served the Southern cause during the Civil War. This was for a variety of reasons, but I think the primary and overriding reason was simply the natural instinct of local people to defend their home territory against an invasion from outside. The Blacks who helped the Confederacy in the Civil War viewed the northern armies as a foreign invasion...and they viewed the "Yankees" as foreign people.
That was also the primary thing motivating the average White southerner who fought for the Confederacy. They were defending their homes and families.
You would find the same sort of thing happening when any country is invaded...and the South did think of itself as a separate sovereign country following secession from the Union.
The vast majority of people naturally rally to the cause when confronted with a threat from beyond their own borders.
Another factor may have been genuinely good relations between some slaveowners and their slaves, in which case the slaves would have been inclined to remain loyal. This was probably the case with Nathan Bedford Forest, since he was a spectacularly good leader of men (if I may judge by his war record).
Now...in the wake of a lost war such as the South's war for independence, you are going to have a lot of bad fallout afterward, and former friends can become bitter enemies. The situation changed radically after the southern surrender. Those who had been in charge in the South were cast down and northern carpetbaggers came in to basically loot the South and fill their own pockets. This stirred up bitterness and hatred that led to the formation of reactionary outfits like the Ku Klux Klan, and that led to all kinds of violent reprisals on Blacks and other targets of that bitterness.
So what I'm saying here is...the fact that the Ku Klux Klan behaved viciously to many Blacks after that war does not necessarily indicate to me that Nathan Bedford Forrest would have behaved viciously to Blacks before that war.
You have to be willing to look at shades of gray in these historical matters, inconvenient as it may be when you want everything out of the past to fit some present modern political position that you attack great emotional importance to...
You have to be willing to imagine yourself in a completely different time, with very different expectations, and realize that had you been born as a White or a Black southerner at that time, you might very well have supported the Confederacy in either case...and NOT because you were consciously supporting what we now term "racism" or even because you were consciously supporting slavery...but for the common reasons of patriotism, honor, duty, and love of your own society and the people around you.
You might have regarded the northern armies with real hatred as you saw them pouring into your home state by the thousands and destroying the entire fabric of the society you had grown up in, and killing the people you knew personally and loved deeply.