Agreed that it would be more intutive if CelestialBodiesShown controlled direct sunlight that hits the 3D world. However right now it is intended to directly control whether the sun’s image is rendered in the sky independent of the amount of light the world receives.
I recommend setting Lighting.Brightness to 0. This will disable direct lighting from the sun. If your world looks too dark after this, try increasing Lighting.ExposureCompensation.