The Book of Mormon teaches that Laman and Lemuel were cursed with a skin of blackness by God.
2 Nephi 5:21
"And he had caused the cursing to come upon them, yea, even a sore cursing, because of their iniquity. For behold, they had hardened their hearts against him, that they had become like unto a flint; wherefore, as they were white, and exceedingly fair and delightsome, that they might not be enticing unto my people the Lord God did cause a skin of blackness to come upon them."

According to the Book of Mormon, this curse is passed down from generation to generation.

2 Nephi 5: 23
"And cursed shall be the seed of him that mixeth with their seed; for they shall be cursed even with the same cursing. And the Lord spake it, and it was done."