The dark line comes from the scan, as for the dark parts they are mountains standing behind the mist which was then desintegrating.
As I used a very swift and light bleach , the highlights are toned and the darker parts remain little affected by the toning.I'd say that this split-toning adds to the mood of the picture by "breaking" the monotony of what would be otherwise a quite uniform background, but that is just an opinion....
Anyway thank you all for your comments.