In a saucepan, combine blueberries, ⅓ cup sugar, lemon juice, and 4 tablespoons of water. Bring to a boil.
Mix cornstarch with the remaining 3 tablespoons of water, then add to the blueberry mixture. Cook until thickened and set aside to cool.
In a bowl, mix cream cheese, 1 teaspoon lemon juice, 1 teaspoon vanilla extract, and ⅓ cup sugar until smooth.
Spread a small amount of the cooled blueberry filling onto each egg roll wrapper.
Layer the cream cheese mixture over the blueberry filling.
Fold the egg roll wrapper into a triangle, sealing edges with a little water.
Heat oil in a deep fryer or pan to 350°F (175°C). Fry the egg rolls for 2-3 minutes or until golden and crispy.
Remove from oil and let cool slightly. Dust with powdered sugar before serving.