Picture becomes all dark and obscured at times

The screen may become dark if Picture mode is set to Cinema or Custom. If the screen continues to remain dark after changing the picture quality mode, go to the Backlight, Picture, Brightness menu and adjust the brightness to your liking. When Power Saving is fixed to Low or High, the screen becomes dark. Some TVs have a Light Sensor so the picture will just become brighter or darker depending on the lighting in the room.
You can also try programming from a different source, like a DVD player or VCR to check whether the problem is caused by the device that sends the signal to the TV.

If the picture mode is set to Cinema or Custom, the screen could become dark. If the screen is still dark after the picture quality mode has been changed, change the setting Backlight, Picture, Brightness and adjust the brightness to your taste. ... Depending on such settings, the picture brightness level can be higher or lower

If Picture mode is set to Cinema or Custom, the screen may become dark. If the screen is still dark after the picture quality mode is changed, change the setting Backlight, Picture, Brightness and adjust the brightness to your taste. ... If Power Saving is set to Low or High, the screen becomes dark
https://www.sony-mea.com/en/electronics/support/articles/00024643

How old is your TV? LEDs get dimmer with time. Plasma has a lifetime, too, as do fluorescent backlights.

When the picture is dark, show a torch light on the screen from a safe distance. If you can see the picture, the backlight is the problem. Anyhow, it is better to show it to a qualified service technician.

It is a rear projection set with 3 picture tubes.

If unplugged and rear cover removed you will see top of chassis 3 large round black cylinders that are the lens for the tubes----they may be obscured with foam padding---clean the lens with a lens cloth or a paper towel with glass cleaner sprayed on the towel.

These are the red,green and blue lens---they are the light source for your picture--if dusty (should be cleaned one time a year) the picture will be darker and dull looking.

Adjust video settings:

1. Press Wega Gate
2. Select Settings
3. Highlight Video
4. Press + to select
5. Adjust/increase the brightness.
6. Press Wega gate to exit.

If picture is still dark, you need to replace the lamp again. Sony recommends that you replace the lamp:

1. After approximately 8,000 hours of use.
2. When the screen becomes dark or the color becomes unusual.
3. When the lamp indicator on the front of the TV flashes.
   

is there any spar, u observed inside the tv when it do like this.

Does the picture just get dark or does the picture go out completely? Does it do this with both the cable and the DVD? Is the audio affected?

The dark rectangle appears because you have the closed captions on and set to TEXT. Closed captions set to CC1 puts the words on the screen and TEXT puts a black box on the screen. Go into the menu, navigate to the page with closed captions and set your captions to OFF.