All of our Blacklight apps except Hyrax (arclight, history, books, espy) just use an SQlite database. Blacklight only uses this so users can see their search history and apparently for the "Next" and "Previous" links. So the db/production.sqlite files are usually small, but they can fill up over time. There is a command to clean old data from the database.
rake blacklight:delete_old_searches