i was just asked to make a vanilla cake with blueberry filling. has anyone ever done this? anything I need to keep in mind when it comes to covering the cake in fondant or keeping it fresh while I decorate it?
Make a precooked blueberry filling thickened with cornstarch, but add more cornstarch to get the filling to stiffen a little more. Cool it to room temperature before adding it onto the cake. Use a buttercream dam the same size as the berries, and chill right after you stack (even before the crumb coat).
When you assemble a cake, you fill it. Then you apply a very thin layer of soft buttercream to set all the crumbs. This then sits until it has crusted, or you chill for 15 minutes. That's the crumb coat. It prevents cake crumbs from getting into your final buttercream coat.
Filling with a dam: take a disposable icing bag of buttercream (no tip) and pipe a border around the cake just inside the top edge. Add the filling inside that. Put the second cake on and press lightly to stick the layers together. If your filling is softish, chill the whole thing until the dam is hard. Then crumb coat, etc.
Sorry--I'm not going to comment on covering a cake with fondant because I only use fondant on fruitcakes. Lots of other good advice on cake central for fondant.