Restoring partially discolored photos

[denzjos]

What I did ( a quick try, not a perfect result but the photo is bad) :
- make a 2 copies from the original layer
- select the color blue on the copied 2 layer using : Colours / Desaturate / Mono mixer (use the sliders to convert the blue to black)
- the picture is a bad jpeg file, so blur the desaturated image (value 2 to 4)
- select the black color : Select / Select by color
- make the desaturated layer invisble to see the copied layer 1 below
- make corrections to the selection with different selection tools
- copy the selection to a new layer
- use Sample points on the selection and the rest of the picture ( the dark colour from the trees - see arrows)
- correct the values of the Sample points with the curves
- correct the selection layer with the levels
- make a new layer from visble
- use the healing tool to correct imperfections  
- Use Colours / Hue-Saturation to optimise or Colours / Rotate Colours
- Colorise if needed