First, give them a unique backstory. Maybe they come from a family that was wrongly accused of dark magic in the past. This will shape their personality and motives. Second, develop their appearance. For a black OC, you could give them dark hair and eyes, perhaps with a scar or mark that has a special meaning. Third, think about their magic abilities. It could be something like the ability to communicate with dark creatures in a way that no one else can. This will make them stand out in the Harry Potter universe.
To create a compelling Black OC, start with their personality. They could be a bit of an anti - hero, having a moral code that is different from the typical wizard. For example, they might believe that some forms of dark magic can be used for good if controlled. Their relationships are also important. Maybe they have a complicated relationship with a Hogwarts professor who is trying to reform them. Also, consider their goals. Are they seeking revenge for their family's past? Or are they trying to prove that not all black magic is bad? These elements will make the OC more interesting in the fanfiction.