In LCD TV at the back there are fluorescent lamps used as light source. In front of these lamps, there are large number of small pixels. These pixels have colour filters . Now if any part of our image is black these LCD TVs are not able to produce complete black colour,instead they will produce dark grey colour. The reason for this is that the colour filters are not able to completely block the back light reaching the screen. So some amount of light reaches the screen and hence we get dark grey colour instead of black colour.