Sounds as though the blinds were installed before the valances were hung. There has to be a way to remove the valance so that you can remove the blinds without breaking the brackets. Are the valances covered with a pleated fabric by chance? If so, gently dig through the pleats and see if you find any screw heads....these could possibly be the way to remove the valance and in turn gain access to the blinds.

Good Luck!