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