Native Americans descended from Israelites who traveled by boat to North America in 600 BCE. God darkened their skin and made them sinful, polluted, blood-thirsty savages to punish Laman (Lehi's rebellious son). Native Americans are the descendants of Laman and are called "Lamanites" in the Book of Mormon.
And he [God] had caused the cursing to come upon them [the Lamanites]... 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. 2 Nephi 5:21
The Lamanites ... were led by their evil nature that they became wild, and ferocious, and a bloodthirsty people, full of idolatry and filthiness; feeding upon beasts of prey; dwelling in tents, and wandering about in the wilderness with a short skin girdle about their loins and their heads shaven.... And many of them did eat nothing save it was raw meat; and they were continually seeking to destroy us. Enos 1:20
The Lamanites ... were a wild, and ferocious, and a blood-thirsty people. Mosiah 10:11-12
The Lamanites ... were ... sinful and polluted. Mosiah 25:11
But at least there was hope for them. If they repented and became good people again, God would lighten their skin, so they'd become white, faiir, and delightsome once more. (You can tell how good a person is by the color of their skin. White is good; black is bad.)
It came to pass that those Lamanites who had united with the Nephites were numbered among the Nephites; And their curse was taken from them, and their skin became white like unto the Nephites; And their young men and their daughters became exceedingly fair. 3 Nephi 2:14-15
And then shall they rejoice; for they shall know that it is a blessing unto them from the hand of God; and their scales of darkness shall begin to fall from their eyes; and many generations shall not pass away among them, save they shall be a pure and delightsome people. 2 Nephi 30:6