From Fedora Project Wiki
Darkserver is a service written to help people finding details of build-id(s). People will be able query the service based on build-id(s) or rpm package names. The service will provide output in JSON format as it will be easier for other tools to parse the output.
The whole idea is based on a feature of BUILD-ID which was completed in Fedora 8 time, you can read about it [here].