MacOS – Black screen at MacOS login

This effect occurs when "Reduce Transparency" is enabled (System Preferences > Accessibility).

(Very odd that this is how the effect would degrade, instead of just make it a solid colored square.)

It honestly seems like you have already tried everything a genius bar would try themselves. However I think I might have some attempts here to help you out.

All boot-up key combinations can be seen from here: https://support.apple.com/en-us/HT201255

1. Remove all connected USB, HDMI, or other plugged in devices including power cable. Try booting again with holding option to select a disk. I recommend doing this after steps 2 + 3, but remove cables first.

2. Lets ensure you are indeed resetting the SMC. This works best without 100% battery, because the light will be orange when charging and the computer is off. While off and the light is orange, hold down shift+control+alt/option and then after holding those three keys hold the power button a moment and release and watch the light change from orange to green to orange again. Volume should also get reset from this. (This can be done without any issues involving firmware password)

3. Clearing NVRAM by holding Command-Option-P-R. NOTE: This will require your firmware password to clear it, but I don't think it will clear with one as I've noticed it doesn't for my MBPr mid 2012. So if you can, remove the firmware password and then try this. But from a command-line terminal the following command will also clear your NVRAM.

Requires Administrator privileges! This is also funky with MacOS Sierra due to new restrictions with their attempts for a new secure system meaning it'll require you to be in safe mode to run it.

sudo nvram boot-args -c

I have more info on boot-arguments if necessary. Just ask.

4. Boot into Internet recovery by holding Command+option+R (Differs from normal recovery because this downloads your original factory release OS recovery rather than using the partition of your current OS on disk.)

5. If it is hardware, lets try Apple's Intel CPU check by holding down 'D' while booting up. This is a hardware test and takes a while.

Let me know how you make out.