John A. Widstoe cites premortal unworthiness that black skin could be a mark.
John A. Widstoe, "Were Negroes Neutrals in Heaven," Improvement Era (June 1944): 385
The opinion is held by many members of the Church that because the negro was a neutral in the great council, held in the heavens before the foundations of the earth were laid, he has been punished with a black skin. There is no evidence, as far as found, to justify this belief. On the other hand, there is ample evidence to support the church doctrine that all who have been permitted to come upon this earth and take upon themselves bodies, accepted the plan of salvation. Those who did not accept it were cast out and became the angels of the evil one. The cause of the black skin of the negro is not known. A mark was placed upon Cain because of his sin. The negroes are supposed to be his descendants. Since these people, themselves, did not commit Cain's sin, it is very probable that in some way, unknown to us, the distinction harks back to the pre-existent state.