Mexicans can be as prejudiced and discriminatory as folks in the USA, or elsewhere. As an African-American woman you've, unfortunately, been exposed to that. I think most Mexicans are totally unaware of the existence of large numbers of blacks in a couple of parts of the country - or about the use of African-origin slaves in Mexico in past history. You'll probably be a novelty, oddity in the colonia where you'll be staying. About safety, pay attention to the advice of the family and you should be just fine. Have a great visit!