Boil 3 tablespoons sugar with vinegar, and 1 cup blueberries in a 1- to 1 1/2-quart heavy saucepan, stirring, 1 minute. Remove from heat.
Combine remaining 2 cups blueberries with peach slices in a large bowl. Toss with hot blueberry syrup and black pepper, then add sugar to taste. Let stand, tossing occasionally, 30 minutes.
