I am not an expert or an historian but I do know as fact that a slave in Texas or Louisiana would have an easier time getting to mexico then to go north.

Here is the only article i have seen written about this matter. I am aware that there are african decent people on the eastern coast of mexico however the issue here is Nayarit is on the western coast (3 hours north of Puerto vallarta). To add, we dont resemble african to the degree of african decent people do in eastern mexican regions or afrobrazilians so that tells me that we have been there longer. http://www.mexconnect.com/articles/1935-african-roots-stretch-deep-into-mexico

I'm a little late to this party, but I found an article you might be interested in. It is titled The Blacks Who Freed Mexico by Ted Vincent. Within the article Vincent says that African slaves were not unknown to Mexico and, in fact, 300,000 to 500,000 slaves were imported into Mexico by 1810. And by 1810 many Afro-Mexicans were of mixed race and comprised 10.2% of Mexico's population. As to how Africans or Afro-Mexicans came to Nayarit, it is most likely due to former plantation slaves, but it should be noticed that there were several Afro-Mexican guerrilla fighting forces in West Mexico in the late 1810s and early 1820s including a group led by Gordiano Guzman in the state of Nayarit. I know this isn't a very in-depth answer, but it does look like information is hard to come by. I would check out Vincent's sources and maybe you can find some more information.