Here's an excerpt from the Blacklight Frequently Asked Questions:
Blacklight is an open source OPAC (online public access catalog). That means libraries (or anyone else) can use it to allow people to search and browse their collections online. Blacklight uses Solr to index and search, and it has a highly configurable Ruby on Rails front-end. Currently, Blacklight can index, search, and provide faceted browsing for MARC records and several kinds of XML documents, including TEI, EAD, and GDMS. Blacklight was developed at the University of Virginia Library and is made public under an Apache 2.0 license.