How to Make It
Squeeze the juice from 2 of the oranges into a strainer set over a small nonreactive saucepan. Add the sugar, mustard and pepper to the pan and bring to a boil over high heat. Whisk in the dissolved arrowroot and stir over the heat just until thickened. Transfer the dressing to a small bowl and chill over ice until cool. Stir in the tarragon.
Meanwhile, using a sharp knife, peel the 6 remaining oranges. making sure to remove all the bitter white pith. Quarter the oranges lengthwise, then slice them crosswise 1/4 inch thick. Place in a medium bowl and add the blueberries. Stir in the dressing.
Arrange the arugula on a serving platter, mound the fruit in the center and serve.