Well, I think your assertion is incorrect. You can make a website that simply forwards the information to the block chain. Basically you could have multiple websites editing the same data / BlockChained 'webpage'. This doesn't make the system any more centralized, since the BlockChain can be edited using a variety of means. It would, however, mean that a rogue website would have the possibility to impersonate you and obviously to pretend it posted your comment while it didn't. I would hope that sheer site credibility would be down the toilet if they did that, and that people could switch to either a browser plugin (which I think has the possibility to be even more nefarious), or whatever means the user feel secure with.

The odd thing about being temper-proof is that it means you can't block any speech at all, but it does not preclude the possibility of mods allowing to post updated versions of posts (deleted or whatever), and, strangely enough, since the source data is always accessible, the user would get to decide just how much moderation he wishes to see.