You know, when Cecil was a Dark Knight, he didn't use a lot of magic. He was all about the attack with his awesome swords. In the FF4 that I'm familiar with, Golbez doesn't do a lot of attacking. He mostly casts all kinds magic.

The only class I can think of that appears in two characters is White Wizard, as Rosa and Porom. They both are proficient in white magic (obviously), and that's what defines "White Wizard". Porom can use "Twin" when Palom is around, and Rosa can use "Aim", but at their core, they use White Magic.

So if you take the only Dark Knight you see (Cecil), and look at what he dominantly does, you can assume that Dark Knights do that. Cecil attacks with his sword. In the non SNES versions, he can use "Dark Wave", but it's not too useful and doesn't accurately reflect his forte.

My guess is that Golbez's class is one you don't see in another character. The closest I can imagine would be the Lunar, as seen in FuSoYa. Golbez seems talented with magic, and is the nephew of FuSoYa, and was not raised as a Dark Knight by King Baron, like Cecil was.