Pause Menu Background

So I have this pause menu GUI script, and I would like that once the pause menu is showned the background would be darker or something, like this:

its like a blue texture

Well open your graphics application and create a alpha channel picture (transparent area) colorize it in black (or any color you want) and lower opacity. Save as png or any other format that supports alpha channel. Use this as gui texture and call it when pause menu is shown up.