So is Seuhyuk and the black hair girl like together romantically or just siblings?

I think they are together, though it's been a while so not sure

Sword Witch Seo Hyejin and Han Seunghuk (who was missing for three years) are romantically involved. When the guild leader in chapter...7? was arguing with people online, she referred to Seunghuk as Hyejin's "Significant other." Seunghuk, when he was talking to the prince, at one point says that he wants to get back to his "Lady Love."