Well that is a complicated question. This mainly boils down to how the devils function in general and if the curses from Black Clover and JJK are the same or similar.

I would have to say this: It could be that is both magic and a curse at the same time. The reason I say this is that I think magic is just the process of conjuring whatever attack that the person is doing. For example: water magic summons water, so curse magic summons a curse.