Where does it say that in, what I'm assuming your referring to, the Book of Mormon?
"...the Lord shall curse the land with much heat...and there was a blackness came upon all the children of Canaan, that they were despised among all people." (Moses 7:8)
"And Enoch also beheld ...the sons of Adam; and they were a mixture of all the seed of Adam save it were the seed of Cain, for the seed of Cain were black, and had not a place among them." (Moses 7:22)
"There is a reason why one man is born black and with other disadvantages, while another is born white with great advantages. The reason is that we once had an estate before we came here, and were obedient, more or less, to the laws that were given us there. Those who were faithful in all things there received greater blessings here, and those who were not faithful received less...There were no neutrals in the war in heaven. All took sides with either Christ or with Satan. Every man had his agency there, and men receive rewards here based upon their actions there...The Bro, evidently, is receivingthe reward he merits" (Doctrines of Salvation published 1954,1:61,65,66)
"...Behold, they had hardened their hearts against him...wherefore, as they were white, and exceeding fair and delightsome, that they might not be enticingunto my people the Lord God did cause a skin of blackness to come upon them. And thus saith the Lord God: I will cause that they shall be loathsome unto thy people, save they shall repent of their sins." (2 Nephi 5:21-22)