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 exceeding fair and delightsome, that they might not be enticing unto my people, therefore the Lord God did cause a skin of blackness to come upon them.

He caused a curse to come on them. It was a bad curse because of their sins. They had made their hearts hard against him. Their hearts were like stone. They were white and very beautiful. But the Lord God made their skin become black. This was so they wouldn’t attract my people.

He brought a severe curse on them because of their rebellion. They had hardened their hearts against him until they were like stone. They had been fair-skinned and attractive, but to keep them from drawing my people away, the Lord caused their skin to become dark.