Fill a small saucepan part way full with water. Add butterfly pea flowers and bring to a simmer. Reduce heat to low, cover and simmer for 10 minutes or until the water is rich cobalt blue.
Place noodles in a heat-proof bowl. Pour over boiling water and let sit for 7 minutes. Return to saucepan over medium heat and boil for 30 to 60 seconds more or until noodles are cooked through and completely clear in color.
Drain noodles and set aside.
For sauce, combine water, lime juice, sugar, garlic, chili and fish sauce in a bowl. Whisk well to combine.
To prepare tofu, whisk together soy sauce, brown sugar, rice vinegar and sesame oil.
Heat a large nonstick skillet or wok over medium-high heat. Add vegetable oil until hot and shimmering. Add tofu cubes. Cook until cubes are just starting to brown, about 3 minutes, then flip and cook for a few minutes more.
Add sauce and bring to a simmer until slightly thickened and syrupy.
To assemble, divide noodles among serving bowls. Drizzle with sauce (the color of the noodles will change from blue to pink). Top each bowl with tofu, along with radish, carrot, fresh cilantro, and a sprinkle of fried shallots and peanuts.