By choosing "black screen" you mean choosing unused input with no signal, don't you?
I dont't think this is good option as all blocks are normally powered ..
If you don't want to watch it just switch the plasma off ..
I believe that switching off (turning off) unused devices is a good way to conserver energy and money. (--> We get an energy crisis in Indonesia .... electricity prices will go high ... so every cent worths!)
By the way, is there any device that shouldn't be switched off even when we're not using it? Because I always turn off all devices at home (including computer) when it is not used.