External Monitor is detected but displays black screen on MBP 15" 2017 I have a 15" LTN156HL01 Lcd display which I turned into an external monitor by using a controller board.
Here's the thing:

It works perfectly on my PS4.
It works perfectly with my brother's MacBook Air.
It even works when connected to my Canon 80D.

But when connected to my MBP 15" the backlight of the external monitor lights up but the image is totally black.
However my Mac detects the screen... even if I change resolution, the problem persists.

Then one day I tried to reboot the MBP in safe-mode and when restarted properly... Hurrah! the screen was actually working super fine.
But when I unplugged it and replugged it... here we are again.. black screen.

With these test, I realized that the monitor works, the controller board works fine and that it can be used with my MBP 15". The problem is not with the hardware, it has to be related with software or OS.
If not, I cannot explain why it worked that time...
What can it be? How can I solve it?

Thank you in advance!
While I'm not sure what you mean by a controller board (is it a video card or a dock?), Here's a few tips to try. 

Test the cable against other cables and inputs. I recently had a mini display cable go bad when it wasn't that old. 

Consider connecting the cable directly to the monitor if you are passing it through other devices (IF Thunderbolt > External HD > Monitor, INSTEAD Thunderbolt > Monitor). 

You could try resetting the NVRAM which would help your mac relearn your monitor. (Command Option P R after a cold 'Shutdown'). Google this for your specific OS.  

Typically, I find a black screen when the resolution of the mac is set too high for the monitor or the refresh rate is incorrect. Match what you know what your graphic card can handle to the limitation of the monitor. If the mac's display setting are vauge, there is a utility called SwitchResX that might be helpful. You can specify the resolution of your monitor and customize it with resolutions, color profiles and shortcuts. It's free to try with some nagware or feature limitations.