Darken skin color

Start by opening an image.

Create selection of the skin.
Press Ctrl+Alt+D to apply feather to the selection and press Ctrl+J to copy the selection in a new layer.

Select burn tool from the main toolbar.
Use settings as shown.
Apply the brush to the whole new layer.

Change the settings of the burn tool. Now use these settings.
Again apply the brush to the same layer.
Change the color mode to luminosity.