Steam green beans, covered, 5 minutes or until crisp-tender. Rinse green beans with cold water; drain. Combine green beans, kidney beans, onion, and mint in a medium bowl.
Combine balsamic vinegar and next 6 ingredients (balsamic vinegar through garlic powder), stirring with a whisk. Pour vinaigrette over green bean mixture, and toss well. Chill for 1 hour. Sprinkle with parsley.