Well, she did say that he was going to be the balance between light and dark magic (I believe). So saying that the magicians of light are of the day, and the magicians of darkness are of the night, that means Jaihi (being ranked as a Dusk magician) is in the middle of them. He's the balance between the two kinds of magicians.
Or at least, that's what I've figured out. Just my opinion.
I figure the same too. Especially since the Guild he belongs to is one of the dark ones with Nine being his mentor.
It's just, the name, a) we get where the title of the series comes from and b) sounds really cool and important.
I betting his scope is bigger than that . Perhaps he is meant to bring light to the dark. Like the guild itself. Just an idea.
Oooh, he met the Goddess herself and she titled him the Dusk Magician. Very interesting.