Blosxom lives at http://www.raelity.org/lang/perl/blosxom/, the Blosxom home page. Here you'll find the latest happenings on the Blosxom front, release notes, links to related projects, and Blosxom-based blogs of particular interest.
The latest version of the script itself is always available for download at http://www.raelity.org/lang/perl/blosxom/download/blosxom/.
Download Blosxom by visiting the Blosxom home page, right-clicking (that's Ctrl-clicking if you're on a Macintosh) the download link and saving the file to your hard drive. Alternately, from the OS X or Unix command line, you can make use of cURL or the like and simply say:
% curl -O http://www.raelity.org/lang/perl/blosxom/downloads/blosxom
With the ubiquitous wget:
% wget http://www.raelity.org/lang/perl/blosxom/downloads/blosxom
Or via the Lynx text-based web browser:
% lynx -source http://www.raelity.org/lang/perl/blosxom/downloads/blosxom > blosxom