I want to create a big nature scene in cycles and want a bluish atmosphere so mountains and distant objects appear bluish. Here is an example of what I'm trying to achieve. Notice how the mountains in the background have a definite blue hue to it. What would be the best way to achieve this? Thanks!
It's possible to do with Volumetrics and other effects, but generally not worth it for both render time and resulting quality. Your best bet would be compositing with Z pass (Z depth) or Mist pass. It would take some tutorials for in-depth compositor nodes for making it inside Blender, but overall the idea is: you render your scene image and another pass(Z or Mist) - a black and white image which goes from black to white depending on the distance from camera to the most distant rendered object. Then you use that pass in compositing the final image with your rendered scene, either inside Blender Compositor with nodes or image editor (like Photoshop).
This tutorial will be helpful! It's for creating mist in Cycles. Cycles will allow you to easily create a photorealistic effect as well.