An interesting branch of this topic is the Confederate agent Jose Quintero and the possible annexation of Coahuila and Nuevo Leon to the Confederacy. Many of the runaway slaves were located in Coahuila...many more relocated to Guerrero and Oaxaca provinces, much further south and deeper into Indian country.
Just as there is currently a Confederate enclave in Brazil, there is a black community in northern Mexico who maintain their traditions, similar to the Confederates. They're the Mascogos - known in the US as Black Seminoles. The Black Seminoles were experiencing a lot of persecution - those with reservations were being sold by their own Indian agents - so the chiefs Wild Cat and John Horse led the Florida people to Mexico, where slavery was outlawed. They have a very intriguing story!
African American Military History by Researcher, Author, and Veteran Bennie McRae, Jr.
lestweforget.hamptonu.edu