Black screen on mac but displays externally? Do I need new display?

Let's check the display again! Using a flashlight shine it through the Apple logo in the lid (a poor mans backlight) do you see the faint images of your desktop and its icons?

If you do your logic board needs its backlight circuit repaired and it might have some corrosion around the display connectors. Here’s the logic board side.

One thing I’ve been seeing a lot as of late is damaged displays as people are spraying their screens with cleaners letting it drip down so the logic at the bottom gets wet which either shorts out the fuse on the logic board and/or corrodes up the Display cable connector on the LCD panels T-CON board.

Replacing the fuse does take a bit of micro-soldering skill and if the logic board its self has damage that too will need replacement parts. At worst you might need a new logic board.

At this point I would hold off ordering any parts!