Mormon closes his discussion of the curse by noting that the Amlicites had not realized that they were cursing themselves. Note the important language: “the Amlicites knew not that they were fulfilling the words of God when they began to mark themselves in their foreheads; nevertheless they had come out in open rebellion against God; therefore it was expedient that the curse should fall upon them.” The reason for the curse was not the marking of the forehead. It was the decision to come out in open rebellion against God. The mark on the forehead was the visible sign of it, but not the cause.
Verse 19 reiterates that they brought upon themselves the curse, but the very fact that they had to mark themselves with red tells us that the rest of their skin pigment did not suddenly change. They were immediately cursed and removed from protective covenants. They were unrighteous, and the “skin of blackness” was the symbolic mark of that apostasy, not a physical sign.