Probably the identifier of the article. A long time ago, we used to use auto-incremented positive integers straight from the database instead of UUID or adjective-adjective-noun.

It's part of the URL path.

schema: http, ___domain: hintjens.com, url path: /blog:99

It looks a bit like a port number, but it's in the wrong place. For that you'd want http://hintjens.com:99/blog