So you're thinking like an African-American Trail of Tears? I don't think it's ASB, but it is severely implausible. Perhaps if you get near constant slave rebellions. Eventually the Southerners may decide that slavery isn't worth the headache, but there is too much bad blood by that point to simply free them. So they get freed under the condition that they leave the existing states so that they can't make any more trouble. The federal government sets up a territory out west solely for freed slaves, like they did with the Natives in Oklahoma. As I said, not very plausible, but given an early enough POD and the right circumstances, it could happen. Definitely not with a post-Civil War POD. You'd probably need a POD in the mid 1700s or so.