There ae people on these boards who will explain this much better than I can, but here goes. When you bless something, it becomes a sacramental, something dedicated to God, blessing the Rosary does not in effect make the prayers better, but you are using a sacramental. There is a book, I think written by Ann Ball on Sacramentals. Hope this helps, you may want to wait for a more detailed and better answer. God Bless.