I don't think blockchain necessarily means anyone being able to make blocks.
It is not a chain, however, more like a tree/web. And anyone can inspect the tree and re-verify its validity.
What is on the tree, however, is decided by me, the webmaster-operator. If there are troll comments that I want removed, they're gone. But I cannot forge something by another user, because I do not have their private key.
And anyone else can fork the forum and append to the tree in their own way, and I cannot stop them.