This is a common misconception in the United States because many American Mormons are from Utah, located in a region of the country that is predominantly Caucasian. For example, notice on the maps below that Utah and neighboring “non-Mormon” states don’t have substantial African-American populations.
That having been said, there are many black Mormons even in the United States. Globally, Caucasians comprise a minority of Church membership and are “losing ground” with each passing year. See my Diversity Slide Show for more. ()