This is what I did.

Roughly mask of the section of shoulder that needs the shadow.  I  used the polygonal lass tool.
Put this selection on a new layer (Ctrl+J).
Lock the transparent pixels on this new layer.
Using the gradient tool choose the colour stop colours of the blend you need.  These would be the shadow colour eyedropped from a shadow area of your picture and the skin tone you want it to blend wtih.
Apply the gradient to the selection.
Change the blending mode of this layer to darken.
With a soft eraser brush remove any excess parts of the selection.
Repaint any hair that was covered up by the blend.

Hope this was helpful.

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