So since SE won't say anything about this,I've been thinking then I realized something.I may be wrong and this may not be the explanation but I noticed how in all of the footage of the Dark Knight where he's wearing the artifact armor set (job specific armor coming soon) he has the black flames only in the job trailer where he is using Darkside.

And then in every other video,pictures and the benchmark,where he doesn't have the black flames even if in combat,he is not using Darkside as you'll notice there is no red aura.

So,could the black flames be an effect from the Dark Knight AF armor when using Darkside? Could it be possible they made an armor set that modifies the graphical effect of a skill?

It would be a big coincidence if I'm wrong.One thing for sure though,it would be nice if SE would finally say something about this.