One possible plotline could be Harry's internal struggle. As he learns dark magic, he has to battle his own moral compass. He might question the boundaries of good and evil. Another could be how his relationships change. His friends might be horrified or try to save him from the dark path.
Snape is a character that could play a major role. Given his past with dark magic, he might be a reluctant mentor to Harry, guiding him in understanding the power and danger of it. Dumbledore, too, would likely be in the story, as he is always concerned about Harry's well - being and actions.
Yes, it can be considered dark and controversial. Blood magic in the Harry Potter universe is often associated with dark and unethical practices. It goes against the principles of the good magic that Hogwarts teaches. For example, the use of blood in dark spells is seen in the actions of Voldemort. In fanfiction, having Harry Potter learn blood magic is a deviation from his character's usual moral stance, which can stir up a lot of debate among fans.
Perhaps it's a magical crystal that enhances his ability to communicate with magical creatures. This would be quite unique as it would give him an edge in dealing with things like thestrals or unicorns, and open up new storylines in the fanfiction. Harry could use this crystal to understand their needs and gain their help in various situations.
One interesting plot could be Hermione's internal struggle. As she delves into dark magic, she might have to fight her own moral compass. She could be tempted by the power it offers but at the same time be haunted by the potential harm it can cause. Another plot could involve her hiding her study of dark magic from her friends, especially Harry and Ron, creating a lot of secrecy and tension in their relationships.
Obviously, Harry Potter is the main character. He is the one who is learning the Dark Arts. Then there might be his friends like Hermione Granger and Ron Weasley. They could be either supporting his new path or trying to stop him. There may also be new characters created specifically for the fanfiction, such as a secretive Dark Arts master who takes Harry under his wing.
Hermione could be a strong ally. She's smart and could help Harry research more about battle magic. Ron too, with his loyalty and courage, would be by Harry's side.
Obviously, Harry Potter is the main character as the title implies he is the one learning magic early. But there could also be Hermione and Ron. Hermione might be intrigued by Harry's newfound knowledge and Ron could be both a bit jealous and in awe of Harry's early magical abilities.
Overcoming limitations is also a theme. Since wandless magic is not the norm in the wizarding world as presented in the original books, Harry has to break through the traditional thinking. He has to deal with the doubts of others and his own insecurities. In some fanfics, he even has to face opposition from the Ministry of Magic who are wary of this unregulated form of magic.
Obviously, Harry Potter himself is a very important character as the story revolves around him learning family magic. His friends like Hermione and Ron might also be important as they could support or help him in his journey of learning this magic.