Yeah, since 99.999999999999999999999999999999999999% of history is pre-1900, the possibilities are almost limitless, unless you're a Calvanists.
Anyways, if we through out Guns, Germs, and Steel type alterations (changing the domesticable animals and grains of Africa) and assume Europe still lucks out like it did historically, you're probably talking about a Mexico (and an Americas) in which the initial contact between Europeans and Native Americans is even more deadly biologically than it OTL was.
Africans were imported into Mexico and other regions of Central and South America as forced labor like in the North, and Carribbean. But basically, Africanized America is basically regions where the natives did exceptionally poorly. Where enough Indians / Native Americans survived to be worked as slaves / peasants, then, there was no need for Africans. (Largely in the South where there were some large precursor native states.)
Where this was not the case, in a Europe dominated world, bring on the enslaved Africans and africanize the area. Anyway, the most likely route to an 'Africanized' mexico is a much worse than OTL depletion of Mexico's native population, either through diseases or genocide, which will pretty much defacto mean a greater import of africans. As it was OTL, the Africans imported into Mexico OTL were simply absorbed genetically into the native and european populations.