2015 Macbook Pro display goes blank but machine continues to run and fans continue to go even if I close lid
You've already reset the System Management Controller and NVRAM (both logical first steps), but there's no mention of you testing the MBP in Safe Mode or of testing your hardware. 

Next I would probably start with testing to see if this behaviour is replicated when booted into Safe Mode and if the issue persists then running Apple Diagnostics.

Boot into Safe Mode 

Follow these steps to boot your Mac into Safe Mode: 


Fully shut down your MBP 
Restart your MBP
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice). 
Take a note of what happens (i.e. does your MBP still behave the same way, etc) 
Exit Safe Mode by restarting your MBP as normal 
Test again when booted normally 


If the problem doesn't occur while booted into Safe Mode, let me know and we'll go from there. 

If the problem persists, proceed with running Apple Diagnostics.

Run Apple Diagnostics

Follow the steps below to run Apple Diagnostics:


Fully shut down your MBP
Restart your MBP
Immediately press the D key and keep it down until you see the Diagnostics screen appear
Wait for Diagnostics to finish (this typically only takes a few minutes)
Once complete, one of two things will appear on the screen:


a No issues found message
a brief description of any errors found plus further instructions

If the diagnostics test does find errors, take a note of what they are


Note: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete. 

Regardless, take a note of what happens and let me know how you went.