this explains a bit about Mexico
In the total population sample, paternal ancestry was predominately European (64.9%), followed by Native American (30.8%) and African (4.2%). However, the European ancestry was prevalent in the north and west (66.7-95%) and, conversely, Native American ancestry increased in the center and southeast (37-50%), whereas the African ancestry was low and relatively homogeneous (0-8.8%).
Most of the European ancestry in Mexicans is from Spain, and there are blonds in Spain although not as numerous as in Northern Europe, so yes, although not common, there will be blond Mexicans from time to time, that do not have recent European or white ancestry, but only ancestry back to colonial times.
I know my ancestors from Spain (my Great Grandfathers parents left Spain for Sicily) were very, very fair with red hair both husband and wife.