Here's why:

* The Sun: Planets get their light from their star. Even if a planet didn't have any moons or nearby stars, the sun would still illuminate the sky with some light, giving it a faint glow.

* Atmosphere: Planets with atmospheres scatter light, making the sky appear blue, red, or even green depending on the composition of the atmosphere.

* Stars: Even planets without a significant atmosphere would still have a starry sky at night, making the sky appear dark but not black.

While the concept of a black sky is a fun idea, it's not scientifically possible for a planet to have a completely black sky.