You should never need this from a Jetson. I do blacklist Nouveau on the PC. Are we talking about a Jetson or a PC? If Jetson, then probably recovery requires flashing again, although you could first save content via clone.
I’m assuming this is on the Ubuntu host you’re using to run the Jetpack on – the Xavier doesn’t use and isn’t affected by nouveau AFAIK.
Press tab when Grub first boots on the PC (or whatever the key is on your machine,) so you get into the Grub menu. There, choose the recovery mode to boot into text mode (or edit the kernel options to boot into text mode.) In text mode, you can remove nouveau from the blacklist.conf again.
Yeah, the Jetson doesn’t use Grub. Then again, it also doesn’t use nouveau. Presumably whatever problem you’re having is actually related to something else. The Jetson uses a module named nvgpu AFAIK.
I’m actually a little surprised nouveau ships on the Jetson kernel at all, but I guess if you plug a NVIDIA card into the PCI-Express slot on the side of the module (assuming AGX here) then it could be useful.
The easiest way out is to put the module into recovery mode (Hold in the center of the three switches, then press and release the reset switch, or power switch if it’s not on yet, keeping the center switch held for about 10 more seconds.)
Then flash the device from your host Jetpack computer as usual.