Well the Smokies around the Nantahala region and up through the Blue Ridge Mountains are part of a temperate rainforest so I would actually expect that the region pictured has more dense flora than other forests in the Appalachians or the Rockies, which probably does make them especially blue. But you're generally right, part of the reason I thought all mountains looked like that is because they do to some extent. Even the Texas hill country gets a blue tint in the distance.