Unfortunately you are many hours away from being able to unlock Dark Knight. It is unlockable once you reach the beginning of the Heavensward expansion, but before then you gotta complete the rest of base ARR (which you seem to be close to doing) and then 80 (yes, 80) more quests that bridge the gap between the end of ARR and the beginning of HW.

As a consolation, they never locked jobs behind MSQ ever again after HW, in fact if you own Stormblood, you can switch to Samurai or Red Mage as soon as you hit 50 in Ul'Dah. From stormblood on, level (and owning the expansion) are the only requirements to unlocking expansion classes.