I finally got the upgrade to work with my Dell XPS 15. Nvidia just released updated drivers last week. I went directly to Nvidia and downloaded/installed the newest reference drivers using the Clean Install option.
Following that, the Windows 8.1 update went smoothly. Before that, I was getting the "boot-to-blank" problem. I was able to roll back those failed attempts. So it seems the issue on the XPS 15 is directly related to video drivers.