this sounds like discolouration of the tubes behind the lcd screen itself. theres very thin flourescent tubes, like in kitchens and offices, behind the lcd screen, these light up white, and the lcd screen changes colour to create the picture that you see
but like the tubes in offices, the filaments in each end can burn the ends of the tubes, and this is the dark marks you see on the sides of the screen
the tubes can be replaced, but its a royal pain in the arse as the tv has to be complatley stripped down, and the fragile tubes that are about 2.5mm wide slid out and new ones put in, without breaking anything or getting any dust anywhere
most people put up with the dark patches untill the tube goes completley, then you have a dark top of the screen, and then they buy a new tv as the prices have fallen over the last 2 years