I run a 2.1 Nevegal in back
I have an '04 Blur with V-brake mounts, so same rear triangle as your bike. I would not run bigger than a 2.1 Nevegal. There's a little room, but not a lot, and if you're packing up mud it will disappear quickly.
Check out this photo of my rear triangle and the Nevegal, it will give you an idea of how much room you have to play with:
I'm not sure I am a fan of the Nevegal. I have been on only 12 or so rides with it, and the blocks have worn nearly 50% already. The tire isn't going to last even a season! It grips incredibly well on loose and rocky surfaces, and wet roots are not a problem. However if your mud is a bit clay-like, it packs up very quickly. I don't have this problem with the Blue Groove front tire though.
I may go back to my Velociraptor rear tire to see how that compares when this tire wears out.
I'm trying to find a large volume tire to replace my Kinetics 2.35. Last year I had to trim all the outter knobs a bit to be able to run it in the Blur, but I'm not willing to do it again.
Is the Nevegal 2.1 a good fat tire ? I know Craig (moderator) has it on his bike, any feedback?
Has anybody else tried a good fat tire that fits on the Bloor tight rear triangle? Mud is an issue (that's why I'm thinking 2.1) but the I'm interested on getting the fattest I can fit.