I get conflicting information. Some sources tell me there was slavery in Mexcio while Wikipedia tells me european colonizers already had African admixture.
Going by the numbers the african was introduced about seven generations ago so around 1840~.
In general, most of the African American ancestry in Latin America stems from slaves imported to the New World. Some of these were imported to Mexico to combat labor shortages caused by disease. The majority were sold from Mexico to traders elsewhere in Latin America, especially Peru. Your estimate on when African slaves were introduced is slightly off though. African slaves were often attached to parties as servants and showed up in the New World in the first years of the 16th century.
I can't speak to your statistics, but there is a great book that in part details African influence in the state of Veracruz - "Flash of the Spirit: African & Afro-American Art & Philosophy” by Robert Farris Thompson. Veracruz is generally considered to be the primary region in Mexico with significant African influence and ancestry, from West Africans brought over by the slave trade.