> I recently cleared my browser history & bingo - ALL the Ztree
> messages are coming up Blue (Unread).
> There's a button up top on the right indicating refresh / mark all
> messages as read.
> How does this work - it does nothing with my display.

The colouring of links in blue (":link") or purple (":visited") is done by the browser, which bases this on your browsing history. Since you just deleted that, the browser has no way of knowing which links have been visited before, and so it shows everything in blue.

Myself, I don’t know any way to restore your browsing history once it’s deleted, but maybe someone else does.

The button ‘mark as read’ is, I think, merely to remove the "NEW" indicator on any messages (I think that’s red; I just read the last message on this forum, so I can’t check right now). That indicator is provided by the forum software, based on when you last visited the forum index page and each of the message pages — while logged in.