Prepare the watermelon by cutting it into cubes.
In a large bowl, mix the diced watermelon with the crumbled feta cheese and the chopped red onion, if using. Incorporate the fresh blueberries for added sweetness.
Drizzle the olive oil over the salad. Gently toss the mixture to combine the ingredients, being careful not to break the watermelon pieces too much.
Once everything is combined, let the salad chill in the refrigerator for about 15 minutes. This resting time will allow the flavors to meld together.
When ready to serve, garnish with additional mint leaves as desired.