Preheat oven to 325ºF. Butter a 2-quart soufflé dish. In a bowl, whisk together egg yolks, butter, buttermilk, lemon zest and juice. Add sugar, flour and salt and whisk until smooth.
Beat egg whites until firm peaks form. Fold one-quarter of beaten egg whites into lemon mixture, then gently fold in remaining whites. Pour batter into baking dish; transfer dish to a large roasting pan. Pour enough hot water into roasting pan to reach halfway up sides of baking dish. Sprinkle blueberries over pudding.
Bake until top is puffed and light brown, about 45 minutes. Remove from water bath and let cool 10 minutes. Sprinkle with confectioners' sugar and serve.