Drain peaches, reserving 2 tablespoons juice. Set peaches aside. Combine peach juice, vinegar, olive oil, and sugar, stirring well with a wire whisk.
Combine peaches, lettuce, and onion in a medium bowl; toss gently. Arrange lettuce mixture evenly on 2 salad plates; top evenly with blue cheese. Drizzle vinegar mixture evenly over salads. Sprinkle with pepper, if desired.