Connecting external display turns MacBook screen black
Could be a hardware related issue (either the MBP or external display) Similar issue happened to me and it turned out to be an issue with the LVDS cable. Also used to work at the Genius Bar, and would see this a lot with certain models of MBP. 

Try running a hardware test https://support.apple.com/en-us/HT201257 , or if you can take it in to the Genius Bar. They have diagnostics which will help isolate it to either hardware or software. 

Have you tried a different external display / display cable?