Preheat an oven to 350 degrees F (175 degrees C). Grease and flour two 10-inch cake pans. Whisk the flour, cocoa powder, salt, and baking soda together in a bowl.
Beat the shortening and white sugar with an electric mixer in a large bowl until light and fluffy. Add the eggs one at a time, allowing each egg to blend into the mixture before adding the next. Beat in the food coloring, 1 teaspoon blueberry extract, and apple cider vinegar with the last egg. Pour in the flour mixture alternately with the buttermilk, mixing until just incorporated. Pour the batter into prepared pans.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 30 to 35 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.
Beat the butter with the confectioners' sugar until smooth. Beat in 1 teaspoon of blueberry extract, then whip until light and fluffy. Once cake has cooled, spread with icing, and sprinkle with chopped walnuts.