Ok. I would remove that first.
Then you could try
sudo pacman -S nvidia-dkms
Reboot and see if no black screen and Nvidia driver installed by posting inxi output. I also ran into trouble with the nvidia-installer script this week. Perhaps @joekamprad can have a look, could be something to do with recently updated Nvidia driver.
Strangely, just installing nvidia-dkms and nvidia-settings worked for me. Then you should be able to use Nvidia on external monitor. Or additionally install Optimus as mentioned above.
(In case the screen goes black, worst case tty and remove nvidia-dkms and reboot via nouveau driver to further troubleshoot.)