Preheat the oven to 325°F (160°C). Arrange the bread cubes/strips on a parchment lined baking sheet in a single layer, drizzle with olive oil, and season lightly with salt and pepper. Bake 12 to 15 minutes or until lightly golden brown. Set aside to cool.
While the bread bakes, combine the watermelon, basil, mint and feta in a large bowl. Let sit, covered with plastic wrap, for 30 minutes and up to 1 hour in the refrigerator.
Combine the balsamic and honey in a small saucepan and bring to a gentle simmer. Cook until the mixture reduced to about 1/4 cup. Season with salt and pepper to taste, transfer to a small glass bowl, and let cool completely.
Before serving, toss the bread and blueberries with the watermelon mixture in the large bowl. Then, drizzle with the balsamic reduction and toss to combine. Check seasonings and adjust, if needed. Serve cold.