Toni, the blue is in the sky right after sunset for about 30 minutes but the camera gathers that in because it's a long exposure with the camera set on a tripod, (since it's already sunset, you can't take a picture without extending the exposure without everything being blurred or the flash going off) your eyes may not see the blue but the camera in that time frame, 30 seconds for Brad's photos, gather enough of the color to bring it out.
The first link I posted from Wiki shows several shots with the blue, the three after the ocean shot have lights on the streets and buildings so you couldn't take those shots without a longer exposure and tripod.
And the elevation shouldn't make much difference other than maybe smog in lower cities adding to the color of the sunsets but we have seen great sunsets up in the mountains.
So all this will just help to make you more aware of the difference in the quality of light, the softer colors of in the morning & the evening colors and it will make you more aware of the twilight.
I did a search on Flickr for Denver sunsets, here are pages of them:
http://www.flickr.com/search/?...
and I picked out a few blue hour ones
City Park, Co
http://www.flickr.com/photos/s...
Denver Metro
http://www.flickr.com/photos/h...
this is a cool sunset reflection
http://www.flickr.com/photos/a...
http://www.flickr.com/photos/a...
http://www.flickr.com/photos/z...
These are from one guy who did several shots and not even with long exposures
http://www.flickr.com/photos/p...
