Combine sugar, vinegar, dry mustard, salt and oil in a small mixing bowl. Set aside. Break off stems from spinach leaves. Wash;drain in colander. Dry on paper towels. Tear spinach into bite-sized pieces and place in a salad bowl. Wash blueberries; drain. Meanwhile, peel and section oranges; cut into thirds. Add oranges, blueberries, and nuts to spinach. Pour dressing over salad. Toss lightly to mix.