the instead filling can be any type of filling . just drain blue berries exceptionaly wwell if they are juicy. you donot want a soogy cake layers.the icing however needs to be firm that is useed on the tops and sides of your are use it for the decorations like the flowers and roses that are on fancy cakes.you can use a buttercream icing stiff for your icing on top and sides. this has eggs in it to make it rich . or use the normal decorators icing with conf sugar if eggs are an issuje or use the royal type icing that gets hard.. the buttercream i mentioned is a classic buttercream and good on cakes. it much better then the frosting we know and is quick to make. i made the classic buttercream for regular cakes,not wedding variety.the filling can be soft as long as it not liquid you be okay. but thnbe icing for the tops should be med constituty for easy spreading . but decorations need med- stiff for flowers. leaves , swags or roses that go on a wedding cake.
csally