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.
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.
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.
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.
Fresh plums might be yellow inside, but cooking them brings out their crimson color. Take advantage of red plums at their prime: Double the ingredients used in the plum mixture (step 3), and spread the extra on toast, or spoon over vanilla ice cream. After cooling the mixture, refrigerate in an airtight container up to 1 week.