Why is the Moon's sky black in daytime?
There is no air to scatter sunlight, so the sky stays black and the stars stay out even at noon.
On Earth the blue sky is sunlight bounced around by air molecules. The Moon has almost no atmosphere, so sunlight goes straight to the ground and nowhere else.
Standing on the Moon at midday you would see the Sun, a black sky and the stars at the same time — although the glare of the sunlit ground makes faint stars hard to see, which is why they rarely show in astronauts' photos.