Thank you for your reply.  I have contacted you with the information (service tag).  I posted not because this is a warranty issue, but rather because it's an FYI post to others who who Dells with both the Intel and Nvidia graphics controllers. It is a SETUP problem that is not a problem with the hardware... rather some sort of conflict between the Intel and Nvidia over which "gets control" of the display monitor.  It took me two weeks to figure it out and it's still not perfect... but now is workable. Here's what I had to do: Soooo... please enter this in your database.. AND I would love it if Dell would send me a solution so that I can start using my TB15 again... Tom
I have a fix for this problem.  My setup xps 9650 with secondary external BenQ 4k monitor connected via thunderbolt port to display port on monitor (I also have a Thunder Dock TB15 which connects the same monitor but right now I'm using a direct cable as described). The problem turns out to be a conflict between the onboard Intel display controller and the Nvidia controller.  The solution is to go to Win10>Settings>System>Display.  Select the external monitor (mine is set to extend the display) and CHECK "make this main monitor".  Since I do 3D work (Blender), I set that program to run on the Nvidia controller. This works for me.... 
XPS 15 9560, external monitors keep turning black