1. Preheat oven to 325 degrees Fahrenheit. Grease and flour a nine x five x three inch loaf pan.
2. Blend flour and cornmeal in a bowl.
3. Beat margarine in a large bowl until light and fluffy. Gradually add sugar and salt. Pour in eggs in a slow stream, then add vanilla.
4. Add dry ingredients, beating just to blend. Stir in blueberries, if desired.
5. Transfer batter to pan. Bake at 325 degrees Fahrenheit until brown, about one hour and 15 minutes. Cool in pan 15 minutes. Remove from pan and cool completely.