The Color of Darkness

We measure light and brightness and the colors that the rays create, but has there ever been a measure, or color frequency of darkness? By darkness, I'm referring to the absence of light, as opposed to a reference to the "dark side" of our nature, for that requires an entirely separate blog.
This topic has been bearing down upon my mind since I observed the true sense of darkness in the sky about a month or so ago. It was an extremely foggy night, after a day of consistent fog. I stepped out my back door to take out the trash, and as I neared the can, and carried out my usual awe of the beautiful Texas hill country sky, was almost stopped in my tracks when I looked east to where the city lights of Austin usually light up the sky. The sky was blacker than I'd ever seen it. The interesting thing about it was that there were other areas of brightness, much nearer and to the north and south, but Austin was DARK. It was incredible to see the sky that looked as if a dark cloak was placed where my usual landmark lies.
How could it be so dark? I wondered. I had never seen it to that degree, and was trying to figure out how it could be so dark, with the reflection of the city lights onto the low-lying clouds. I've not quite reached my conclusion yet, even though we had the same circumstanced the next night. I've been so perplexed that I've been observing the color of darkness since then.
How dark is my house? How dark is a moonless night? An early morning ride to school?
Since that night, I check every night to make sure I wasn't imagining things that dark night, and am usually reassured by the glow of the city.