What does the story of Estebanico tell us about race relations in colonial Mexico?
I assume that you are talking about the African slave who is known by various names, including Esteban, Estebanico, and Estevanico. He is the man who was part of the Cabeza de Vaca expedition through North America. I would argue that he shows that blacks could, in very rare circumstances, gain some status, but that they would always be seen as inferior in colonial Mexico.
Estevanico gained what status he had through his actions on the exploratory trip made by Narvaez and Cabeza de Vaca. He was very useful to the expedition when they were in extreme circumstances--when they were shipwrecked and trying to make their way back to Mexico through unexplored North America. In other words, it took very unusual circumstances to allow Estevanico to get any status.
Even though he had this status, he was not freed. When the expedition made it to Mexico, Estevanico was not freed or honored in any way. Instead, he was sold (or given) to the Viceroy.
This shows that even if a black person could gain some through success in extreme circumstances, he could not become anywhere near the equal of a Spaniard.