The only reason some people don’t believe in Black Magic is because they have not been affected by it. They might ask for logic, but no logic works for someone who does not believe in what’s written in Quran and Hadith. These people are not victims of Black Magic, they are victims of ignorance and lack of knowledge.
If someone is seeing something out of the ordinary they should start reading manzil immediately once in the morning and once at any time after maghrib (or isha). They should blow 3 times on a glass of water and drink it.