Combine the blueberries, water, lemon zest and sugar in a medium saucepan. (Mixture will seem very watery, but do not worry; the cake will absorb the liquid as it cooks.) Bring to a low boil on the stovetop. Simmer for 3 minutes, stirring gently. Pour into a 9- by 13-inch pan.
Mix the batter ingredients until smooth. Gently spread the batter over the fruit, covering most of the pan with batter. (It is OK if the batter does not reach the edges of the pan.) Bake at 375° for 25–35 minutes or until golden brown on top. Serve warm or room temperature with ice cream, whipped cream or milk.