Just as info, both nvidia and ati force as default behavior the loading of their kernel module. Starting in init 1 has no bearing on this, since the module is loaded at boot, basically, or shortly after that process starts, so whoever is posting that advice might want to update their methods and actually test what they are suggesting, it takes a lot more than that to block radeon or nouveau kernel modesetting modules, sadly.
sgfxi has all the blacklisting stuff required automated, including rebuilding initramfs, which is also needed usually.
To blacklist radeon kms you need to add this to the grub boot line, sgfxi does this correctly when you install fglrx or nvidia, but if you have to get into a session with no radeon running, edit the grub kernel line and add: radeon.modeset=0 nomodeset
the nomodeset is just to be one the safe side, but you need the radeon.modeset one, or nouveau.modeset=0
This stuff is hard to keep up with because the stuff changes almost every year, at least it has so far, but that's where it's at now.
There's a reason sgfxi has many hundreds of lines of code dealing just with modeset blacklist and blacklist removals, this stuff is a royal pain the butt to deal with, but last I checked, sgfxi should do the job fairly smoothly, definitely for nvidia, which is now thoroughly checked, and I think for fglrx too, at least last time I checked it. I believe also the debian nvidia driver, at least, will blacklist nouveau, which requires a reboot, but I'm not totally sure about that, but I do know sgfxi does, in several ways, because one hasn't been enough to block that gunk.
Like other users here, however, I don't put myself in a situation where I have to use ati cards unless I can use old ones, then the radeon driver from xorg is just fine, my preference in fact, but that's for older ati cards, much older.