vanilla custards with roasted blueberries

INGREDIENTS

• 1 cup whole milk
• Seeds from 1/4 to 1/2 vanilla bean (see Note) or 1 teaspoon pure vanilla extract
• 3 large egg yolks
• 1/4 cup granulated sugar
• 3 tablespoons all-purpose flour
• 1 to 2 tablespoons (see Note) unsalted butter
• 1 cup blueberries
• 1 1/2 teaspoons granulated sugar
• Juice from a wedge lemon, or to taste