Some things I noticed: 

 - onGUI and onLevelWasLoaded should be **OnGUI/OnLevelWasLoaded**
 - You forgot to set the red component here, so you are not setting the alpha value of the color
> GUI.color = new Color (GUI.color.g, GUI.color.b, alpha);
 
 - Try to set the *alphaBlend* parameter to true on the DrawTexture function (I think it's true by default, but I didn't test it, so give it a try):
> GUI.DrawTexture(new Rect(0, 0, Screen.width, Screen.height), fadeOutTexture, ScaleMode.StretchToFill, true);

 - You shouldn't change the alpha value in the OnGUI function, it's better to do in the Update