Put the almonds, sugar and mint in a food processor and pulse briefly to make a coarse streusel-like topping. Transfer to a small bowl.
Combine the grapefruits and oranges in a large decorative bowl. Combine the reserved citrus juices in a small saucepan (you should have at least 2 cups, if you don't, add enough fresh orange or grapefruit juice to make up the difference). Add the honey and fresh ginger and bring to a boil. Cook until the mixture is reduced by half. Remove from the heat and let cool to room temperature, at least 30 minutes. Remove the ginger and pour the mixture over the citrus. Cover and refrigerate the salad for at least 1 hour and up to 24 hours.
Just before serving, stir in the blueberries. Top with the minted almond sugar and serve.