Aside from slaves escaping to the USA, I would expect that there will be 'maroon' colonies of internal escapees developing in more remote areas. Certainly the Confederate authorities will do everything they can to exterminate them, but I believe that some will survive in one form or another. These are not going to be Revolutionay Socialist states as in TL-191, but tight-knit communities of internal escapees, some indigenous peoples, and even some renegade whites. Brazil provides a number of these examples. This will of course add to the militarization of Confederate society and I believe also serve to keep the CSA internally focused and not adventuresome in foreign affairs. Even considering this, I see no strong reason to suggest that the CSA will last, although de facto slavery will need to be abolished between 1890 and 1910 due to international pressure. Just my random thoughts, but I did want to introduce the 'maroons' here.