Cut watermelon in half. Using a melon baller, ball out flesh of watermelon into a large bowl.
Add blueberries and mozzarella. Mix well.
Scrape out any remaining flesh of the watermelon. Reserve to serve salad in if desired (alternatively a bowl can be used)
Balsamic Glaze
In a small saucepan over medium high heat, boil the balsamic vinegar until it has reduced to about 1/4 cup and coats the back of a spoon. This will take about 10 minutes. Monitor so it does not over reduce.
Cool to room temperature (or chill) before adding to the salad. Glaze can keep refrigerated for about a month.
Drizzle salad with balsamic glaze, if using. Garnish with basil if using.