Heat an outdoor grill to high, or heat a griddle pan on the stovetop.
Cut the watermelon into half-inch thick wedges, remove the rind, and use 1 tablespoon of olive oil to brush onto both sides of the wedges.
Cook watermelon on each side for 2-3 minutes until grill marks develop.
Arrange the grilled watermelon on a serving plate and top with feta and blueberries.
In a small bowl, mix together olive oil, lime juice, orange juice, and salt, then drizzle over the salad.
Garnish with mint leaves and serve immediately.