How to apply blur effect only to part of the image

Hello,

I am looking for a way to blur only a specific part of the image/screen. Let's say I would like to blur the bottom-left quarter of the screen. Is there a way to do that?

I don't know much about writing shaders and googling for such thing yields no usable results. I assume that coordinates specifying the blur region would be specified by internal shader variables so the affected region could be easily adjusted.

Any suggestions are greatly appreciated!

edit: Here's an example of what I'd like to achieve.