Make lemon curd: In top of a double boiler or in a heatproof bowl, whisk together lemon zest and juice, sugar, and eggs. Add butter and set over (not in) a saucepan of simmering water. Cook mixture, whisking constantly, until thickened, about 20 minutes. Strain through a fine-mesh sieve into a bowl. Put in an airtight container and chill until ready to use.
Make batter: In a large bowl, combine flour, sugar, and salt. In a large glass measuring cup, whisk together eggs, milk, and vanilla. Gently whisk wet ingredients into dry ingredients until everything is thoroughly blended. Cover and let stand at room temperature 1 hour or, preferably, chill in refrigerator 2 to 3 hours.
About 20 minutes before end of resting time, preheat oven to 450°. When oven is very hot, put in a 10-in. cast-iron skillet for 10 minutes to thoroughly preheat. Working quickly, with all ingredients at hand, pull skillet out of oven with oven mitts and set on stovetop or a heatproof trivet. Pour in melted butter and swirl pan (use caution; it will spatter). Whisk batter to smooth it out. Pour batter into skillet, scatter blueberries over the top, and return pan to oven.
Bake until dutch baby is puffed up and golden brown around edges and set in center, about 15 minutes. Sift with powdered sugar. Divide among plates and top with lemon curd.
Make ahead: Lemon curd, covered and chilled, up to 2 weeks. Batter, covered and chilled, up to 3 hours.