This blog has no built-in search functionality. Instead users are supposed to download the full blog source code and search inside it. This way they are fully in control of the search and in most cases it should not leak the search to a third party.
Here is an example that works under a terminal in GNU/Linux with git installed:
git clone https://git.replicant.us/contrib/GNUtoo/infrastructure/haunt-blog
And then you can search in it. For instance if you want to search for FOSDEM in the blog articles, you can use the following commands:
cd haunt-blog
git grep -i fosdem -- markdown