LCD back lit is always active as long as the set is on, it's the LCD it self that produces black.

For fluorescent backlit yes. Not true for LED. That's how they get "dynamic" contrast range and power savings, by turning off the backlighting in areas of the screen where it isn't needed.

 

U are saying that part of the strip shuts off, in places where it is black.
Only this way black can be dynamic compared to white and shades of grey

 

The set has several HDMI outputs and a USB input for service and reading JPEG files.

That USB port almost certainly provides power, 5V across the two outermost pins. That's enough to run a small fan outright (a bit slow, if it's a 12v fan) and more than enough to trip a relay.