Whisk together vinegar, orange juice and salt. Add oil in a slow steady stream, whisking until emulsified.
Put salad greens, blackberries, feta and walnuts into a bowl. Drizzle with vinaigrette, and toss gently. Season with salt and pepper.
Sugared Walnuts: Preheat oven to 350°F Combine sugar and salt in large bowl.
Bring a small saucepan of water to a boil. Add walnuts and blanch for 2 minutes. Drain and immediately toss with sugar/salt mixture.
Transfer walnuts to a rimmmed baking sheet, and bake , stirring occasionally, until golden brown, about 10 minutes. Let cool. Sugared walnuts can be stored in an airtight container at room temperature for up to 3 days. Make about 2 cups.