Are there any African American Mormons?

According to LDS Apostle Melvin J. Ballard p. 21, Three Degrees of Glory;" Black people are black because of their deeds in the preexistence and are not permitted to receive priesthood and the ordinances of the temple to prepare them to enter the Celestial Kingdom" Do modern Mormons still believe that? What is preexistence? What is the Celestial Kingdom ?