To an astronaut , why does the sky appear dark instead of blue?
Sky appears blue when seen from the earth because of scattering of light by air & dust particles.
To an Astranaut , since there is no atmosphere in space, sunlight passing through the surrounding space fails to get scattered, hence appears black. There are no mass particles that can either absorb, scatter or even reflect light. There are no masses agaisnt which light can bounce of. So for an astronaut, he is looking at emtpy space through which light is passing by. Hence the space around will appear black.