The blue sky appears due to the scattering of atmospheric particles, whereas there isn't any atmosphere in outer space. The light gets refracted in different directions due to the presence of colloidal particles in the atmosphere, which is the reason for the scattering of light. The reflection of blue light on our eyes makes the sky appear blue. The phenomenon of scattering of light isn't possible due to the absence of atmosphere in space. Due to this, the sky appears dark to an astraunaut.