Greg, you can darken the sky by moving the blue channel slider to the left when you convert to mono using CC. The trouble is that the mist is also reflecting blue sky light as well. However, if you use the Saturate/De-saturate tool (grouped with the dodge and burn tools) to de-saturate the mist first, you can darken the sky without darkening the mist. The mist will still need some dodging with a soft brush to lighten it further but it will begin to stand out. A bit more contrast and a vignette to darken the corners will add a bit of punch. A good way of adding the contrast in a controlled way is to use the Unsharp Mask tool but reverse the position of the first two sliders. The effect of all of this looks like this - if that is what you had in mind (I have also cropped it slightly). Hope that helps.