Q:

Why does the LCD show only black in Live View in manual mode on T3i, until I half-press the shutter? 

I am trying to use my T3i in M mode and also use live mode.   I don't seem to get an image on the LCD screen unless I press the shutter button 1/2.   As soon as I take the picture the LCD screen goes back to all black with a white rectangle outline in the center of the screen.   I will be using the camera in an underwater housing and seeing the live view before I am ready to snap the shutter will be important to me. 

A:

The Live View screen will by default try to 'mimic' the exposure you're likely to get with the settings you have dialled in.   So in dim light with ISO 100, a fast shutter speed, and a small aperture, you will likely just see a black screen.   When you half press the shutter the camera brightens it up while it meters the scene and so you can see what you're focussing on.   
Next time you have this issue just try increasing the shutter speed up through 1 sec, 4 secs, 10 secs etc and you should see the brightness of the image on the lcd display increase. 
In a way it's just telling you that your image will be seriously underexposed!!   As you say, when you take the picture, it's just showing you a black screen again (the image preview).   
So boost your ISO, use a larger aperture, use a longer shutter speed....whatever it takes to get the correct exposure for the environment you're shooting in :-)

