"So the light is a SECRET??? You won't let on?"
Sorry, LaFoto--didn't mean to be cryptic. This was shot in fairly late afternoon. If you shoot a blue sky with a polarizing filter and convert with the channel mixer, this is about what you get. Boosting the red channel makes the sky even darker. Trees with white bark, such as sycamores and cottonwoods in my area, stand out nicely against the dark background. I had this approach in mind when I took the shot.
Myopia, thanks for the feedback. The tint is pseudo-selenium, done with a color curve. I like it but tastes vary.