One way to achieve a soft, moody look in a painting is to ease the transitions across the edges.
Instead of losing the edge by blurring it, the idea here is to keep the edge sharp, but lighten up to the tones on one side of the edge and darken down to the edge on the other. The edge is there, but it feels softened.
For example, in the Fritz von Uhde painting "Schwerer Gang" above, the roofline of the building is lightened as it approaches the realm of the air. The forms are almost lost as their edges meet the sky. Likewise where the base of the trees and the figures meet the ground, they seem melted into the dark earth.
There’s almost no chroma to this painting, but it has a great feeling of light thanks to the handling of transitions.
J.M.W. Turner’s watercolors partook of this quality more and more as his career progressed. In this picture called “The Dark Rigi,” The mountain range is sharp only at the top, and blurs off at the sides. All the contours are suppressed by means of value transitions. The boats at lower right are absorbed into the blue vapor of the water.
One must guard against getting too carried away with this device, however. Too much of of this “fiberfill treatment” can take the backbone out of a picture, so it should be used deliberately and with purpose.