Richard D. Draper, S. Kent Brown, and Michael D. Rhodes identify "seed of Cain" with "skin pigmentation."
Richard D. Draper, S. Kent Brown, and Michael D. Rhodes, The Pearl of Great Price: A Verse-by-Verse Commentary (Salt Lake City: Deseret Book, 2005), 126
the seed of Cain were black: This is a second reference to skin pigmentation as a point of social differentiation. The earlier reference concerns the people of Canaan whose skin color brought spite upon them (see Moses 7:8). There is reason to believe that, because the narrative features the people of Canaan and the people of Cain in different versions, these peoples are different.