1. Heat oven to 425 degrees F. Grate 8 Tbsp.(1 stick) butter on big holes of a box grater, place back in freezer. Melt 2 Tbsp. butter and set aside.
2. Wisk together milk and sour cream. Refrigerate until needed. In mixing bowl, wisk flour, sugar, baking powder, baking soda, salt and lemon zest. Add frozen butter to flour mixture and toss with fingers until throughly coated.
3. Add milk mixture to flour mixture; fold milk into flour with rubber spatula until just combined. Transfer dough to liberly floured work surface. Dust surface of dough with flour. With floured hands, knead dough 6 to 8 times, until it holds together in a ragged ball, adding flour as needed to prevent sticking.
4. Roll dough into aprox. 12″ square. Fold dough in thirds, like a business letter. Lift short ends of dough and fold into thirds again, to form aprox. 4″ square. Transfer to a lightly floured plate and chill in freezer 5 min.
5. Transfer dough to floured work surface and roll aprox. 12″ square. Sprinkle blueberries evenly over surface of dough, then press down so blueberries are slightly embedded. Roll dough (may have to use a bench scrapper or spatula to help lift dough) pressing to form a tight log, 12″ X 4″ rectangle. Use a sharp knife, cut rectangle crosswise into 4 equal rectangles (I would call them more like squares). Cut each rectangle (or square) diagonally to form 2 triangles and transfer to parchemnt lined baking sheet.
6. Brush tops with melted butter and sprinkle tops with sugar. Bake until tops and bottoms are brown, 18-25 minutes.