As the name suggests, Black Bird Manipulation allows the user to imbue crows with cursed energy and control them at will. The user can manipulate multiple crows at the same time from an extended range away from themselves. Additionally, the user can also share vision with the birds, making the technique perfect for surveillance. The crows are absolutely obedient, so they can also be used offensively, even at the cost of the bird's own life.
Earlier in her life, Mei Mei believed that her innate technique was weak because it allowed her to manipulate weak birds and that was it. She dedicated herself to being effective without the technique until she reached her physical limit. Hitting this wall allowed Mei Mei to return to focusing on developing her innate technique, allowing her to master it and flourish into a grade 1 sorcerer.
During the Shibuya Incident, Mei Mei used several of her crows to investigate the inside of Meiji-Jingumae Station before sending her team inside. They flew around each floor of the subway station, giving Mei Mei a live view of what's happening inside. Her crows allowed her to discover that most of the civilians were trapped on B4F and B5F. They were being attacked between B1F and B2F as well, allowing Mei Mei to figure out the curtain that's preventing them from entering the lower floors originated there.
Against the Smallpox Deity, Mei Mei only had a single crow to use. This turned out to be just enough as Mei Mei exorcised the special grade disease curse with Bird Strike, her best attack with Black Bird Manipulation.