Grill watermelon, covered, on a greased grill rack over medium-high direct heat until seared, about 5-6 minutes per side. Remove from heat; transfer to platter; chill for about an hour
To serve, top chilled watermelon with tomatoes, arugula, blueberries and feta. Drizzle with olive oil, season with salt and pepper. Drizzle with balsamic glaze.