I had the same problem, so I tried your solution, and it worked! Thanks!
More specifically, on reaching the black screen with the “_” mark in the upper-left, I typed Ctrl-Alt-F3 to get into TTY3, then I logged in as root, then I typed the following:
pacman -Syu linux66-nvidia-open linux612-nvidia-open linux615-nvidia-open nvidia-settings
After that completed, I used “systemctl poweroff” to power-down, then on starting the machine, the GUI Linux login screen appeared, and on logging in, the desktop came up normally. 
Oddly, of the 3 computers I ran this upgrade on, only one of them had this issue. Maybe it’s because the problem computer uses nvidia graphics? Of the other two computers, one uses ATE graphics and the other use intel graphics.