@glakhbir Firstly Welcome.
There were some notes I read online about issues with Sony 4K TV’s that I managed to find again. Sony say that in the TV If Picture mode is set to Cinema or Custom, the screen may become dark. If the screen is still dark after you change the picture quality mode, change the setting Backlight, Picture, Brightness and adjust the brightness to your taste
There is also a walkthrough for optimal 4K CoreELEC settings Here