The material of certain objects in the scene doesnt show up. Also the scene is tainted dark, is this related to the sky texture as background? How do I correct this?
When combining several outputs into some other inputs, you should remember what socket you use and how.
In your case, you don't see any textures in Rendered view because information from Color outputs couldn't be converted properly being plugged into Shader inputs.
And the blue color fullfilling everything in your case is the color of the environment texture (which is added by the Sky Texture node).
Instead you should mix colors, textures, etc using nodes designed for that, i.e. Color > MixRGB one. Its ouput is also of Color type, so you can continue adding some color information to the node tree.
In the very end, when you know you're done with color, plug the final Color output into the Color input of the Diffuse (or another one) shader. After that, you may combine shaders (add some Glossy or Transparent or adding new colors / textures with new Diffuse shaders or whatever) with Mix Shader node, but there won't be any conversion between Color and Shader type.
You may want to look at meaning of the color of the nodes sockets for more info on different sockets' types.