In a medium bowl, stir together the graham cracker crumbs and 3 tablespoons of sugar. Mix in the melted butter. Sprinkle evenly into the bottom of a 9 inch square baking dish, and pack down into a solid crust.
In a large bowl, beat cream cheese with 1 cup of sugar until smooth. Stir in salt and lemon juice. Fold in the whipped topping until well blended, then fold in the frozen blueberries. Spoon over the crust in the baking dish, and spread evenly. Cover with plastic wrap and refrigerate for at least 1 hour before slicing into squares and serving.
Frost is a common problem with wild blueberries and is the primary cause of a poor harvest. Not all blueberry farms have the same climate. The fields in Quebec and Maine blossom before the fields on Prince Edward Island and are therefore more susceptible to frost. The later blossoming of Island berries usually occurs after the last frost of the spring.