Why doesn't it get dark right after sunset?
Sunlight still reaches the air high above us and scatters down, fading slowly as the Sun sinks further.
Civil twilight lasts until the Sun is 6° below the horizon; you can still read outside. Nautical twilight runs to 12°, and astronomical twilight to 18°, when the sky is fully dark.
The blue of twilight is partly thanks to ozone high up, which soaks up orange light.