Even sweeter and juicer than fresh, grilled watermelon plays well with sweet and sour blueberries and salty feta cheese and mint ties together all of the flavors. Serve the salad right away otherwise the salt from the feta will draw out too much water from the fruits and turn the salad into a less-than-appetizing soup.
Preheat the grill or grill pan to very high heat. Brush the watermelon slices with olive oil. Grill for 2 to 3 minutes per side. Cool slightly and cut into 1-inch cubes (makes about 3 cups). Add to a serving bowl with the blueberries, mint, vinegar and feta. Toss to combine. Serve chilled.
If cooking on a grill, leave the rind on the watermelon slices, as it will be easier to maneuver with tongs.