Coarsely chop frozen blackberries, and place them back in the freezer until ready to stir into the batter. To freeze muffins, let cool completely, and place in zip-top plastic bags. Reheat in microwave 20 seconds.
Work quickly to spread the warm frosting over the first layer, stack the second on top, and then spread the remaining frosting over the top and sides before it sets. If you prefer, you can substitute walnuts for the pecans in this luscious cake.