Yep the atmosphere is transparent to all colors of light except blue. It doesn’t absorb blue light but it does get scattered. That’s why no matter where you look in the sky, you see blue. Because the gasses there are scattering blue light in all directions, including toward you.
Good question. No idea. If higher frequency correlates with more scattering, then UV should also be scattered, right? Or is it something about blue visible light being just right for our mix of gases? Interesting question since that mix has changed over time. Perhaps the sky was another color during life’s early years. Is that why they’re called the Cyanobacteria? Because they turned the sky blue?