This is the specific blessing that follows Mormon’s description in the previous verse that Alma “blessed the earth for the righteous’ sake.”
Notice that the “blessing” actually takes the form of a “cursing.” The important point here is that it occurs for the benefit of the righteous. The particular cursing of the earth is attached to those “which do wickedly.” It may be that after the revelation of the ultimate destruction of the Nephite nation that Alma sought some affirmation that while there would be a victory over the Nephites, that there would not be a victory of evil over good. In this prophetic blessing, Alma promises that the wicked will not prevail, and thus this becomes a blessing for the righteous in that it promises ultimate salvation, if not more immediate victory.