1/4 cup plus 6 tablespoons sour cream
Preheat oven to 400°F. Pierce potatoes with fork. Place on oven rack; bake until cooked through, about 1 hour 15 minutes. Transfer to baking sheet; cool 5 minutes. Cut off top third of each potato. Scoop flesh from bottoms into bowl, leaving 1/4-inch-thick shell. Scoop flesh from tops; add to bowl. Discard tops.
Add 1/4 cup sour cream, 1/4 cup blue cheese, butter, garlic and rosemary to potato flesh; mash. Season with salt and pepper. Transfer mixture to pastry bag fitted with large star tip; pipe mixture into potato shells, dividing equally. (Can be prepared 1 day ahead. Cover and chill.)
Preheat oven to 400°F. Bake potatoes on baking sheet until heated through and beginning to brown, about 25 minutes. Spoon 1 tablespoon sour cream atop each potato. Sprinkle 1 teaspoon blue cheese over each and serve.