Comment 3 for bug 1848326

I had the same problem (the system got upgraded from 19.04 to 19.10, nvidia-430 driver).

I noticed that nvidia drivers were loaded in the rescue mode - so they were likely not blacklisted properly.

When I did not enter the rescue mode I had the following message displayed:
"A start job is running for udev Wait for Complete Device Initialization (1min 3s/ 3min)

/var/lib/gdm3/.local/share/xorg/Xorg.0.log contained the following (see the full file attached):

[ 47.771] (==) ModulePath set to "/usr/lib/xorg/modules"
[ 47.771] (II) The server relies on udev to provide the list of input devices.
 If no devices become available, reconfigure udev or disable AutoAddDevices.
# ...
[ 47.778] loading driver: nvidia
[ 47.882] (==) Matched nvidia as autoconfigured driver 0
[ 47.882] (==) Matched nouveau as autoconfigured driver 1
[ 47.882] (==) Matched modesetting as autoconfigured driver 2
[ 47.882] (==) Matched fbdev as autoconfigured driver 3
[ 47.882] (==) Matched vesa as autoconfigured driver 4
[ 47.882] (==) Assigned the driver to the xf86ConfigLayout
[ 47.882] (II) LoadModule: "nvidia"
# ...
[ 48.142] (II) Unloading vesa
[ 48.142] (EE) modeset(G0): drmSetMaster failed: Permission denied
[ 48.142] (EE)
Fatal server error:
[ 48.142] (EE) AddScreen/ScreenInit failed for gpu driver 0 -1

I removed the nvidia driver and got the system to the working state again and then installed it from scratch.

Then I set up a trace point in prime-select here:
https://github.com/tseliot/nvidia-prime/blob/cf757cc9585dfc032930379fc81effb3a3d59606/prime-select#L126-L138

Tracing showed that /lib/modprobe.d/blacklist-nvidia.conf and /lib/udev/rules.d/80-pm-nvidia.rules were created correctly this time:

https://paste.ubuntu.com/p/2Dp5jgFQty/

cat /lib/modprobe.d/blacklist-nvidia.conf
# Do not modify
# This file was generated by nvidia-prime
blacklist nvidia
blacklist nvidia-drm
blacklist nvidia-modeset
alias nvidia off
alias nvidia-drm off
alias nvidia-modeset off

I could then successfully reboot without hacking.