Until the 1978 revelation to Mormon President, Spencer W. Kimball, black men were not allowed to hold the priesthood. This was
based, in part, on Genesis 4:15, where God placed a mark on Cain after he had killed his brother Abel. Joseph Smith and
later LDS leaders interpreted the "mark upon Cain" to be turning of Cain's skin (and that of all his descendents) black.
It should be noted, however, that Mormons were not alone in this interpretation of Gen.4:15. Many Protestant
denominations at the time Joseph Smith founded the LDS Church (1830) had the same racist understanding of the "mark of
Cain." (See link 3)