Butter inside and lid of 12-ounce coffee can or cylindrical pudding mold. In place of lid, butter piece of parchment paper or aluminum foil.
In mixing bowl combine cornmeal, flours, baking soda and salt. Add maple syrup and buttermilk, mix well. Stir in blueberries and lemon zest. Pour batter into can or mold, top with lid or paper. Secure paper around can or mold by tying kitchen string tightly partway down side. Place can or mold on rack inside tall pot. Add enough boiling water to reach halfway up side of can or mold. Place pot over low heat and simmer 1½ hours. Check water often, adding more boiling water as needed to maintain level.
Remove can or mold from pot, uncover and check center of bread with toothpick. If toothpick does not come out clean, recover can or mold, replace in pot and steam 5 minutes. Transfer can or mold to cooling rack. When bread has pulled away from sides of can or mold, tap bread out onto rack to cool. Slice bread into rounds and serve with Boston baked beans.
Makes one loaf.