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.
Recipe courtesy of Min Kwon