In a small saucepan, add enough water to cover the noodles by about 1 inch. Add the butterfly pea flowers and bring to a simmer. Reduce heat to low, cover, and let simmer for 10 minutes until the water is a rich cobalt blue.
Place noodles in a heat-proof bowl, pour over the blue boiling water, and let sit for 7 minutes. Return to medium heat and boil for 30 to 60 seconds more, or until noodles are clear (follow the noodle package instructions but use the blue water).
Drain noodles and set aside.
For the sauce, combine water, lime juice, sugar, garlic, chili, and fish sauce in a bowl. Whisk well and set aside or refrigerate.
For tofu, whisk together soy sauce, brown sugar, rice vinegar, and sesame oil. In a skillet over medium-high heat, add vegetable oil, then tofu. Cook until starting to brown, about 3 minutes per side. Add sauce and simmer until thickened.
To assemble, divide noodles among bowls. Drizzle with sauce, allowing some noodles to change from blue to pink. Top with tofu, radish, carrot, cilantro, shallots, and peanuts. Serve at room temperature.