"Feathering Off" when using Oil Paints

This is my second layer for the background.  Even though you want the background soft, it still has to have depth and highlights.  The trick is feathering off the lights into the darks without losing the tones.  I use a medium tone to do this (a medium-dark Ultramarine Blue) brushing along the edge of the dark.  I can then gently brush in towards the dark area without lightening it, and consequently brush outwards (after washing my brush out) into the light area without darkening it.  You can always go back and redarken and clean the highlights further when it is dry.