Cook beans in a 6- to 8-quart pot of boiling salted water, uncovered, until crisp-tender, 4 to 5 minutes. Drain in a colander and immediately transfer to a bowl of ice and cold water to stop cooking. When beans are cool, drain and pat dry.
Whisk together oils, vinegar, mustard, salt, and pepper in a large bowl until combined well, then add beans and sesame seeds and toss to coat.