Preheat the grill to medium heat. Brush the cut sides of the peach halves with olive oil and sprinkle with 1/2 teaspoon of salt.
Place the peaches cut-side down on the grill and cook for about 4-5 minutes, or until grill marks appear and the peaches are slightly softened. Remove from the grill and let cool slightly.
In a large bowl, combine the mixed salad greens, fresh blueberries, and remaining 1/2 teaspoon of salt.
Slice the grilled peaches into wedges and add them to the salad mixture.
Gently toss the salad with the balsamic vinaigrette, then sprinkle with crumbled feta cheese and walnuts, if using.
Serve immediately and enjoy!