I got the same problem when im sending client info - i fix the problem by moving the mouse all the time while joining the server... Pretty annoying tho
Hmm... On the game you want to go on (not gmod coz i dont want to spazz it up) try putting in -fullscreen in the launch options. (Right click on the game icon, go on properties, then launch options)
It worked for me when my steam games got blackscreen...