What causes blue sky or white clouds?

Rayleigh scattering -- blue light scattered preferentially by N2, O2; sizes << wavelength  of light. Red sky at sunset/rise due to long path so most blue light is already scattered away

H₂O droplets in clouds scatter independently of wavelength, so they generally look white/grey/dark. More droplets/scattering causes them to not let light get through. The thicker ones look dark, but they are really just darker than the sky. It is the contrast that makes them look "black".