Believe it or not, there were few blacks on the show, but they were mainly extras.
If you don't believe me, go to the website: www.bookguy.com/Mayberry/BlacksInMayberry.htm
There was an African American actor by the name of Rockne Tarkington who made an appearance on the Andy Griffith Show episode titled "Opie's Piano Lesson" and he plays Flip Conroy.
There are a few examples of Mayberry cities as far as I know. You will find a number of them in New England, most towns in the Pacific Northwest and throughout the United States.
The Mayberries of California as far as I know are Lodi, Ripon, Escalon, San Luis Obispo, Santa Barbara, Tehachapi, Mojave, Needles, some cities in the San Joaquin Valley and much of the Sacramento Valley.