Some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. 2 Peter 3:16
And the LORD set a mark upon Cain. Genesis 4:15
Although there is nothing in the text of Genesis 4:15 that suggests that the mark of Cain in involved a change in skin color, it was commonly believed in the 18th and 19th centuries that God cursed Cain with dark skin, and this was often used to justify racism and slavery.
This interpretation is made explicit, however, in the Mormon scriptures, which says "the seed of Cain were black" in Moses 7:22 of the Pearl of Great Price.
But if the mark of Cain was a black skin and it was passed on to his descendants, then how could there be black people today, since only Noah (who was a descendant of Shem) and his family survived the flood? Well, the Mormon Scriptures have the answer to that too.
According to Abraham 1:21-25 in the Pearl of Great Price, Ham (one of Noah's sons) married a woman named Egyptus who was a descendant of Cain. And so the seed of Cain (and black skin) was preserved through the flood.
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 descendants) 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."
