How can I have a small black texture strech over the entire screen inside my gui?
I want to make sure nothing from the past level is visible when i show a load screen.
And creating a huge back texture seems a bit pointless 
It is pointless. Watch my first ShaderLab video to learn how to make something black. Apply that to some mesh, or a Line Renderer.
Nice tutorials you have there.
Hmm so I guess the best way is to make a prefab thats a black mesh with a camera looking at it and instantiate it before loading a new level.
Figured out an easy way to do what I want, creating a custom style with a black image as normal background image.
I can put a loading text centered in there, or a loading image.