Dressing: In a screw-top jar, combine blueberry vinegar, oil, salt, and pepper. Cover; shake well. To assemble: In a large bowl, combine greens, blueberries, walnut halves, crumbled cheese, and snipped chives. Pour desired amount of dressing over salad. Toss lightly to coat. Divide salad mixture evenly among 4 individual bowls.
Blueberry vinegar: In a stainless steel or enamel saucepan, combine 1-1/2 cups blueberries with 2 cups rice vinegar. Bring to boiling; reduce heat. Simmer, uncovered, 3 minutes. Stir in 2 Tbsp. honey. Remove from heat. Pour mixture through a fine-mesh strainer and let it drain into a bowl. Discard berries. Transfer strained vinegar to clean 1-quart jar or bottle. Add another 1-1/2 cups blueberries to the jar or bottle. Cover tightly with a nonmetallic lid. Store vinegar in a cool, dark place for up to 6 months. Before using vinegar, discard berries. Makes 3-1/2 cups.