You can darken a daylight one. If you load my Panoramic shader, the HSV node let's you control brightness. Change the "Value" parameter to alter the sky luminance. Note that a long-exposure night photo reveals the same blue sky as in the day. Night sky and day sky are actually identical - just different brightness.

You can find some actual night ones at cgtextures.com in the Skies 360/Night section. (Note: The images at cgtexures are domes, not spheres. Use the EnvDome.)

To pick up the general lighting, use IDL in P8 or PPRo 2010.

If you don't have IDL, use my GenIBL to generate a matching IBL for use in P6, P7, or PPro.

To mimic sun or moon, you want one infinite light. Positioning it so it lines up with sun/moon in the photo will cause it to match. I made a thread on how to do that somewhere. Perhaps somebody can link you to it if I don't find it right away.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)