Place 2 tablespoons of the sugar and almond paste in the food processor and process until well mixed. Add the salt and flour and place the mixture in the freezer for 1 hour.
Place the flour mixture and the butter in the food processor and pulse until half is the size of peas and the other half is like oatmeal. Turn the mixture onto the work surface and make a well in the center. Add the ice water, a little at a time, just until the mixture holds together. Roll the dough on a well-floured surface until it is a 12-inch diameter circle. Place on jelly roll pans.
Combine 3 tablespoons of the sugar and almonds together and sprinkle 1 tablespoon of the mixture onto the center of the circle of dough to within 1-inch of the edge of the pastry. Place in the refrigerator while you assemble the filling.
In a bowl, toss together the blueberries together with the remaining 4 tablespoons sugar.
Preheat an oven to 375F. Remove the pastry from the refrigerator. Spread the blueberries over the center of the pastry, distributing evenly, leaving a 1-inch border uncovered. Fold the uncovered edge of the pastry over the plums, pleating it to make it fit. Bake until golden brown, 30 to 40 minutes. Remove from oven and cool for 5 minutes, slide the galette off the pan and onto a serving platter.
To serve, cut the galette into wedges and serve with a scoop of ice cream alongside.