The main feature of Blogsym is multilingual support; blogging in two (or more) languages at once is relatively simple. See http://ola.rinta-koski.net/blog/ for an example of a blog created with Blogsym in Finnish and English.
For better or worse, Blogsym doesn't depend on any other Lisp package, so you can run it on a vanilla CL. The development is done on OpenMCL.
Find Blogsym at http://ola.rinta-koski.net/blogsym/
Why not just use CL on the server?
Why indeed? Well, PHP is a far lower common denominator than CL when you look at a typical web server installation. Blogsym is not intended as a solution for the case when you can serve the pages directly out of CL.