Earlier this evening I was looking at the Moon through my cheap toy telescope (x150 magnification) when I noticed a (rather mundane) optical effect I couldn't explain. The Sun had just dipped below the horizon and the sky was still a fairly bright shade of blue, and the Moon itself near zenith. But when I looked at the Moon through the aperture, the backdrop of the sky is notably blacker. My question is what part of the telescope causes this shift in the apparent color of the sky? The best explanation I could come up with is this
Possible Explanation: The objective lens is recessed by a small distance into a hallow cylinder, which reduces the amount of visible light being gathered. So when I look at the sky through the aperture, it's really the same color (wavelength) just dimmer, causing the apparent blackness.
However, while I'm somewhat confident the above is correct, it's disappointingly vague and qualitative in nature. I'm extremely rusty on my basic geometric optics and so I'm curious if there are other factors which could be understood on the basis of simple reflection and refraction, lenses and mirrors, etc. Can anyone help me refine my understanding of this very basic physical phenomenon?