Udon noodles get their flavor here from a toasted sesame seed sauce.
Bring a stockpot of water to a boil. Stir in the noodles, and cook until al dente, about 6 minutes or according to package instructions. Remove from heat, and drain. Transfer noodles to a serving bowl, and toss with 1 tablespoon sesame oil. Set aside.
Gently toast unhulled sesame seeds in a small skillet set over low heat until just fragrant, about 2 minutes. Transfer seeds to a bowl to cool.
Place toasted sesame seeds, soy sauce, mirin, the remaining 2 tablespoons sesame oil, and the rice-wine vinegar in the jar of a blender; process until smooth and creamy. Pour dressing over reserved noodles, and toss to combine. Sprinkle with black sesame seeds, if using, and scallions. Serve immediately.
Unhulled sesame seeds, available at Asian markets, will make the sauce creamy; the plain white variety will not produce the same results.