1. Put yeast in the bowl of a stand mixer (or large mixing bowl) and pour 1 cup warm water (90° to 105°) on top. Let sit until yeast dissolves, 5 to 8 minutes.
2. Add milk, 1/4 cup oil, the kosher salt, and 1 tbsp. rosemary. Attach dough hook and with mixer on low, gradually blend in 6 cups flour. Beat on medium speed until dough is smooth and elastic, 10 to 12 minutes; if necessary, add about 2 tbsp. more flour until it's only slightly tacky. (Or knead by hand: Stir in 5 3/4 cups flour with a spoon, then knead on a work surface, adding flour as required to prevent sticking.)
3. Cover dough in bowl with a towel or plastic wrap and let rise at room temperature until doubled, 1 1/4 to 1 1/2 hours. Oil a 12- by 17-in. rimmed baking sheet with 1 tbsp. oil. Punch down dough, transfer to baking sheet, and use your hands to push it into an even layer; if it's too springy, let it rest about 10 minutes. Cover loosely and let rise until puffy, 45 to 60 minutes. Meanwhile, preheat oven to 425°.
4. With your fingers, poke holes 1 to 2 in. apart straight down into risen dough. Scatter blueberries over dough, then drizzle dough with 2 tbsp. oil. Combine remaining 1 tbsp. rosemary and the sugar; sprinkle on top.
5. Bake until golden, 25 to 30 minutes. Brush with remaining 2 tbsp. oil and sprinkle with flaked salt. Cool in pan on a rack 15 minutes. Loosen focaccia with a wide spatula and slide onto a work surface. Cut into pieces about 3 by 4 in. and serve warm or at room temperature.