Question

why sky look black instead of blue to astronuts

Answer 1

Due to the absence of air molecule or fine particles  as they go very far from earths atmosphere so there is hardly any scarcity of light . This is the reason why the Astronauts see the sky black instead of blue from a very high  altitude. 
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Answer 2

Answer:

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the bluelight has been scattered out and away from the line of sight .

Explanation: