Make the crepe batter: Whisk together milk, 1/2 cup water, 2 tablespoons melted butter, and the eggs in a medium bowl. Whisk in flour and salt; set aside.
Make the sauce: Stir blueberries, sugar, butter, lemon juice, cornstarch, and salt in a medium saucepan over medium-low heat. Bring mixture to a low boil. Reduce heat; simmer, stirring often, until berries begin to break down and release their juices, about 10 minutes. Set aside.
Make the filling: Puree cottage cheese, cream cheese, sugar, and vanilla seeds in a food processor. Transfer to a medium bowl; stir in blueberries. Set aside.
Make the crepes: Stir together remaining 3 tablespoons melted butter and the oil in a small bowl; reserve 1/4 cup for cooking blintzes. Heat an 8-inch nonstick skillet over medium heat. Lightly brush pan using remaining butter-oil mixture. Pour a scant 1/4 cup batter into pan; swirl to form an even layer. Cook until bottom is lightly browned, about 1 1/2 minutes. Using a heatproof spatula, flip crepe; cook 30 seconds. Transfer to a plate. Repeat with remaining butter-oil mixture and batter.
Transfer a crepe to a clean work surface. Spoon 2 heaping tablespoons filling onto crepe 1 inch from bottom and sides. Fold bottom over filling. Fold in sides, and roll up. Set aside, seam down. Repeat with remaining crepes and filling.
Heat the reserved 1/4 cup butter-oil mixture in clean pan over medium heat. Working in batches of three, fry blintzes, turning once, until golden and crisp, 2 to 2 1/2 minutes per side. Place two blintzes on each plate. Spoon warm sauce over top, and dust with confectioners' sugar.
When forming blintzes, be careful not to overfill each crepe. That way, the creamy mixture is less apt to seep out as you fry the blintzes, making for a neater presentation.