1. Make crust: Preheat oven to 350 degrees. In a food processor, grind graham crackers, almonds, and 1/4 cup sugar until finely ground; add butter, and process until moistened. Transfer to a 9-inch removable-bottom tart pan. Using the base of a dry measuring cup, press mixture firmly into the bottom and up sides of pan. Freeze at least 15 minutes.

2. Make filling and bake: Carefully wipe processor blade and bowl clean. Place cream cheese, sour cream, 1/2 cup sugar, egg, vanilla, and salt in food processor; blend just until smooth. Place tart pan on a rimmed baking sheet; fill with cheesecake mixture. Bake until filling is just set, 30 to 35 minutes. Transfer tart pan to a wire rack to cool completely.

3. Meanwhile, make topping: In a medium saucepan, combine plums, 1/2 cup sugar, and lemon juice. Cook at a rapid simmer over medium-high heat, stirring frequently, until mixture is jamlike and moves around pan in a single mass when stirred, 15 to 25 minutes (time will depend on ripeness of fruit). Reserve 1 tablespoon cooking liquid (no solids) for blueberries; cool remaining plum mixture.

4. Assemble tart: Leaving a 1-inch border, spread cooled plum mixture over tart. Reheat reserved plum liquid (on stove or in microwave) until liquefied. In a medium bowl, combine with blueberries, and scatter on top of plum mixture. Chill until ready to serve, at least 2 hours and up to 1 day. Remove from pan before serving.