Focusing further on the Birch Ballet painting, the distant shrubs can be further blended and faded-out by lightly applying pastel pencil over the soft pastel already applied to the painting.
In this example, a pastel pencil is applied with and against the direction of the soft pastel, enhancing the tangled look of the shrub line and softening the edges to convey distance.
Instead of using a tortillion (a paper stick) for blending, consider a pastel pencil or a hard pastel to do the work.