I will like to point out the reason why Dark Knight may not have a class is because it is a Advance Job.

Yoshi-P mentioned back in 2.0 he has plans for Advance Jobs that will require may be 2 Jobs to unlock such as the example he used being Red Mage requiring both Black Mage and White Mage.

Though I question how a Dark knight will work as a Tank since in past FF lore the Dark Knight was known for being a DD Job with tank like stats that was used to keep itself alive while it used its own HP to perform High DD level damage.