Rhetorical: Jacob is once again using structural opposites to make his case. It is tempting to look at verse 9 as simply a prohibition against color prejudice. This is absolutely a correct principle, but not the highlight of Jacob's argument. The key to understanding what Jacob is doing is to note that in verses 9 and 10 he contrasts the fathers of the Lamanites to the children of the Nephites. In this parallel opposition, Jacob stresses that the unrighteousness of the current Lamanites may be laid at the feet of their fathers (Laman and Lemuel) but that the evil visited upon the children of the Nephites is to be laid at the feet of the Nephite men (their children's fathers).
The reference to skin and filthiness is a reference to unrighteousness (symbolic for however much it might have had a physical antecedent). This filthiness of the Lamanite fathers is placed in the past, with the current Lamanites in a more righteous position. This is contrasted to the current "filthiness" of the Nephite men, even though their fathers were clean and "white."
The imagery flowing through Jacob's discourse continues to rely upon a reading of filthiness and skin of darkness as terms connoting unrighteousness. Whatever physical changes may have come upon the literal Lamanites after their separation from the literal Nephites, the physical signs were turned into linguistically connoting symbols marking their position of unrighteousness before the Lord, an unrighteousness that derived from their attempts to kill Nephi, and perhaps a subsequent abandonment of many of the teachings of the Law of Moses (although this is not clearly known for this time period).