In a medium-sized saucepan, over medium-low heat, simmer orange juice until reduced to 1/4 cup and syrupy; cool slightly. In a blender container, combine reduced juice, half of the blueberries, the honey, lemon juice, sugar, mustard and salt; blend until smooth. With the blender running, gradually drizzle in oil until a thick dressing forms. Cover and refrigerate until service. (Makes about 2 cups.).
Core and quarter apples; slice quarters crosswise. In a large bowl, combine apple slices, pecans, celery and the remaining 1 cup blueberries; cover tightly and refrigerate until needed.
At service: Blend dressing to loosen. Arrange spinach leaves on cold salad plates; top with blueberry-apple mixture, dividing evenly; drizzle with dressing.