Despite being a bit outdated with the contents in the last section (this guide was from 2013), most other sections are actually still valid. It is a very good, thorough guide that has helped a lot of people.
Black Mage DPS Guide
Guide in 'Disciples of Magic' published by Gigarit, Apr 4, 2015.
Detailed BLM guide with DPS rotation, videos and endgame boss fight information.