Dark Knight starts at level 30 with all the skills up to level 30.
On my Dark Knight Job Skill List I have all the skills, on that list I should have Darkside (30) between Power Slash (26) and Grit (30), but there's no Darkside skill... I didn't even noticed that I didn't had that skill until I got to level 34 and got Dark Dance (34), that can only be executed while succumbing to the Darkside. I went looking for Darkside on the Job skill list, and it's not there.
As I said before, everyone else that I talked to and play Dark Knight has this skill (Darkside), I don't.