Hey, I'm sure there are many other threads made of this very same issue, but to make it more seen, I want to throw my own into the bunch.
I started having issues right after installing the 6.2 patch files through the FFXIV Steam launcher that I usually play through.
I pressed the Play button as usual, but was greeted by nothing but a black screen. If I tried clicking on the screen, the black screen FFXIV would then instantly crash.
I reinstalled the game. It would give me the black screen again. This time, I decided to check out the Steam Overlay settings and tried to mess around with them, ultimately enabling them. For extra security, I launched the game directly from my Steam library too. The game booted up fine and I was able to do stuff normally.
EDIT: After much testing, the game also boots up just fine if I have the overlay disabled and I start the game by pressing the small icon on my desktop, so not through Steam library.
I've come to the conclusion that the black screen is entirely random of an occurrence for me, not dependant on anything.
One thing that keeps on persisting, that bothers me, is when I try to exit the game after the 6.2 patch after the black screen issue started emerging, the screen fades to black as normal, but the game refuses to shut down until I actually click the screen. Essentially, I have to force crash the game in order for it to shut down completely.
Issue is most definitely connected to the Steam Overlay change they made in 6.2.
I remember vaguely something like this happening years ago for me with FFXIV.