Boil 3 cups water in a medium saucepan. Stir in rice and reduce heat to medium-low. Simmer, covered, until most liquid is absorbed, 40 to 45 min. Drain excess water. Fluff and let cool slightly.Toast nuts in a non-stick frying pan over medium. Stir often until golden, about 5 min. Set aside to cool.Whisk vinegar with Dijon, maple syrup, salt and oil in a small bowl. Add onion and let stand for at least 10 min.Stir onion and vinaigrette into warm rice. Stir in blueberries and nuts. Serve warm or chill in refrigerator for at least 1 hour before serving.