why does the sky appear dark instead of blue to an astronaut

The sky appears dark instead of blue to an astronaut because there is no atmosphere in outer space to scatter sunlight. On Earth, the atmosphere contains air molecules and tiny particles that scatter shorter wavelengths of light, especially blue light, in all directions, making the sky appear blue. In space, without an atmosphere to scatter the light, sunlight travels in a straight line and does not reach the astronaut's eyes in scattered form, so the sky appears black or dark to them.