No worries, it's recoverable without reinstalling.

1. Boot to a text console without SDDM

In GRUB press e on the openmamba entry, append this to the end of the line starting with linux, then press Ctrl+X:

systemd.unit=multi-user.target module_blacklist=nvidia,nvidia_drm,nvidia_modeset,nvidia_uvm

This skips the graphical login, and blacklisting all four modules keeps the console on the firmware framebuffer instead of NVIDIA's (your list was missing nvidia_drm).

If the screen is still black, also add nomodeset. Or, if sshd is enabled, log in from another machine: the system boots fine and networking comes up in multi-user mode.

2. Update the driver first, then remove it

sudo dnf upgrade nvidia_470
sudo dnf remove nvidia_470

The upgrade to nvidia_470-470.256.02-9mamba matters. That release regenerates the initramfs on removal, dropping the nouveau blacklist and the NVIDIA modules from it. Older releases left them in, and you would boot again with no working driver.

3. Reboot normally

Without extra boot parameters the system will use nouveau, and Plasma 6.8 on Wayland will work.

About going back to Plasma 6.7: sorry, that's not an option. 6.8 is a full stack update, and upstream has dropped the X11 session (kwin-x11). If you really need the 470 driver (e.g. for performance on your Kepler card), the alternative is LXQt on X11, as mentioned in the announcement.

Neither

systemd.unit=multi-user.target module_blacklist=nvidia,nvidia_drm,nvidia_modeset,nvidia_uvm

nor

systemd.unit=multi-user.target module_blacklist=nvidia,nvidia_drm,nvidia_modeset,nvidia_uvm nomodeset

worked. I just got black screens.

Instead I logged in blind: pressed alt+ctrl+f1, then entered my username and password, then

sudo dnf remove nvidia_470 -y && reboot

Note that nvidia_470 was already up to date.

After some waiting, the system rebooted and no more black screen after logging in!

Not quite. Now I the screen flickers to black whenever I move the mouse cursor. Web browsers seem to fail to render anything and are unresponsive. This makes the system not usable. I have done some web searching and I believe the issue is caused by either wayland, nouveau or the combination of them. Do you know this issue?

I am thinking I should switch to lxqt but I am not sure what causes this issue, maybe I could do a less drastic change.

Glad you got back in, and thanks for the links.

Yes, it's an upstream KWin 6.8.0 regression with nouveau, tracked in KDE bug 527328. It hits several older NVIDIA cards on nouveau (GT 520, GT 710, GTX 750, Quadro K4200...), while the same setups were fine on 6.7.5. There's no fix from the KWin developers yet.

Workaround

Reporters on the bug say these variables remove the flicker. Create the file ~/.config/environment.d/99-kwin-flicker.conf with:

KWIN_USE_BUFFER_AGE=0
KWIN_DRM_FORCE_GL_FINISH_PRESENT=1
KWIN_FORCE_SW_CURSOR=1

Then disable the "Shake cursor" effect: System Settings → Accessibility, or add this to ~/.config/kwinrc:

[Plugins]
shakecursorEnabled=false

Log out and back in, or reboot. The variables must be set this way, because kwin_wayland --replace from a terminal doesn't pick them up. One reporter still sees a single blink when windows are minimized or closed, but the system should be usable.

Browsers

This may be the same issue, or nouveau's OpenGL acceleration on these older cards. Try again after applying the workaround. If they still don't render, disable hardware acceleration in the browser settings to check (in Chromium-based browsers you can also start them with --disable-gpu).

If it still flickers, please post your card model (lspci -nnk | grep -A3 VGA), and consider adding your hardware to the KDE bug: more reports help get it fixed sooner. We'll pick up the fix as soon as it's released.

I have not attempted changing those variables, I have moved to lxqt but I noticed today that this black-screen flash/flicker is not exclusive to kwin, it happens in sddm too. I think it's just Nouveau and Wayland. It'll probably go away when switching to x11, but I thought I'd report this here.