The Holy Knight's Dark Road: Volume 1 by Yusaku Sakaishi

Young Sain Forth has it made; as the famed Holy Knight, he has the people’s love, the respect of kings, and the blessings of a literal goddess. His aspirations lie elsewhere, though. With his assistant Melia at his side, he leaves for an academy in a foreign kingdom, where he is treated as a deviant by his peers.
It’s his own fault, mind you—his stubborn insistance on wearing a custom, all-black uniform makes him stick out. He isn’t the only outcast in his class: Alicia, born into the Clan of Light but unable to use anything but fire magic, faces similar struggles, and the two quickly become allies.
Alicia proposes that they enter the labyrinth below the school in search of a holy sword, but Sain has other plans in mind: a doom sword. And so, with his friends in tow, Sain ventures into the perilous unknown, driven by a wish to become the coolest, edgiest, most awesome thing in existence: a Dark Knight.
There’s just one problem. He can’t use dark magic. Like, at all.