Purpose: To explain why you might be getting a black screen when sharing your screen on discord
Summary:
This guide is made when you run into an issue sharing your screen. For a specific example this will be targetted toward Discord, although this fix can be applied to sharing your screen with an Apple TV or in a meeting(Zoom, Teams, Meet...etc.).
The Culprit:
In my experience, it seems that hardware acceleration seems to limit your ability to share your screen for media.
"Hardware acceleration is the process of transferring some of the app processing work from the software that runs on the central processing unit (CPU) to an idle hardware resource, which can be a video card, an audio card, the graphics processing unit (GPU), or a special device like an AI accelerator, to optimize resource use and performance."
Turning off Hardware Acceleration:
Below are ways to turn off hardware acceleration. For the browsers and discord, by deactivating the setting will require you to restart the application. Beware that if using a meeting software it may cause you to leave the meeting and have to rejoin to continue.
Chrome:
Click the hamburger menu in the top right.
Then click 
In the search bar at the top, start typing 
Uncheck the slider for graphics acceleration and relaunch Chrome
Firefox:
Click the hamburger menu in the top right.
Then click 
In the search bar at the top, start typing 
Uncheck both checkboxes and restart your browser to try again.
Discord:
Click the settings icon
Click the tab 
Uncheck the slider for Hardware Acceleration
Then confirm the setting to relaunch Discord
Concluding Notes:
Some websites require hardware acceleration to load their embedded media. If a video or image is not loading on a site, follow these steps to enable hardware acceleration to see if it fixes that issue for you.