Toss arugula, blueberries, watermelon and feta together. Drizzle with balsamic right before serving.
Alternatively, you can reduce ½ cup traditional balsamic vinegar over medium heat until reduced to half.
This arugula salad is best served day that it is made, as the watermelon will start to water down the salad if left assembled for too long. Another option is to assemble all the ingredients in advance and toss together right before serving.