Macbook pro boots to a black screen after a few seconds of use
The video is not very clear to understanding the issue. It seemed like you clicked on the “Ansel Admin” username and it blanked out. Then you clicked the power button and it came back to login screen. Also, it will help if you describe what you are doing and the problem (in the video as a voiceover :).

Here’s what you can try.


If you have a monitor available, check if you can see the display after logging in.


There are two ways I found to increase screen brightness (as you mentioned that it was not fully blacked out). (Links to original articles below). 

Method 1: Using osascript, key code 144 (brighter), 145 (dimmer)

$ osascript -e 'tell application "System Events"' -e 'key code 144' -e ' end tell'


$ osascript -e 'tell application "System Events"' -e 'key code 145' -e ' end tell'


Method 2: Using Homebrew - https://brew.sh

If you have homebrew installed, you can install brightness to control.

$ brew install brightness


$ brightness 1


$ brightness 0.5


I have tested the osascript commands on Catalina 10.15.4. You will need to give the terminal accessibility permissions.

If these don’t work, try booting in Safe Mode. 
If it resolves the issue, it may be some startup item that could be causing this.
Hold down the Shift key when powering up and you hear the startup chime to enter Safe Mode.

Links:

How can I dim the screen from Terminal?

How to Adjust Screen Brightness from Terminal in macOS
https://www.maketecheasier.com/adjust-screen-brightness-from-terminal-macos/