In a medium-sized saucepan, over medium-low heat, simmer orange juice until reduced to ¼ cup and syrupy; cool slightly. In a blender container, combine reduced juice, half of the blueberries, honey, lemon juice, sugar, mustard and salt; blend until smooth. With blender running, gradually drizzle in oil until a thick dressing forms. Cover and refrigerate until serving. (Makes about 2 cups.)
Core and quarter apples; slice quarters crosswise. In a large bowl, combine apple slices, pecans, celery and the remaining blueberries; cover tightly and refrigerate until ready to serve.
Blend dressing to loosen. Arrange spinach leaves on cold salad plates; top with blueberry-apple mixture, dividing evenly; drizzle with dressing.