Hi Falconius. I'm going to assume you're using blender.
The pitch black shadow is the result of your lighting setup and your render settings:
You seem to have only one light source in your scene. The type of light, the power, colour, position, rotation and scale, also your project scale will all affect the final look of your entire scene, especially with just one light. If my assumption is wrong, your other light source is either way too weak or its visibility is turned off.
EEVEE rendering engine does not calculate light path bounces = it gives you no ambient light = darker shadows.
I avoid booleans, so I'm not sure if there's a setting you missed, or it's just acting buggy. If this was my scene to work on, I would simply apply the boolean, then go into edit mode and fill the missing faces.
