Step 1In a large, nonreactive bowl, combine the blueberries, blackberries, lemon juice and sugar. Let the mixture rest for 2 hours at room temperature for the juices to develop. Shortly before serving, toss the berry mixture with the chopped mint. While the salad is resting, prepare the fool.
Step 1In a small saucepan, place 1 cup of the blueberries and one-fourth cup sugar over medium-low heat. Cook, stirring occasionally until the sugar has dissolved and the berries have softened, about 10 minutes. Remove from heat and transfer the berries to a medium bowl to cool completely. (To speed the process, chill the berries in the refrigerator for about 20 minutes or about 5 minutes in the freezer.) Add the remaining cup blueberries to the cooled blueberries and set aside.
Step 2While the berries are cooling, in the bowl of a stand mixer or in a large bowl using a hand mixer, whip together the cream, remaining 3 tablespoons sugar and vanilla to form soft peaks. Add the creme fraiche and continue beating until it is combined and the mixture again forms soft peaks.
Step 3Using a large rubber spatula, gently fold the blueberries into the whipped mixture, being careful not to overmix. (You want to create a marbleized effect.) Refrigerate, loosely covered, for up to 2 hours before serving.
Step 4To serve, spoon about one-half cup of the fruit salad into each of 6 dessert bowls, and top with a generous portion of the fool. Garnish each portion with a mint sprig.