I think most people believe in black magic because people like to blame other people or things for their problems. If they have bad luck then some force is working against them. Another reason people believe in black magic is because thoughts are powerful. If you think you are cursed then you act like you are cursed. Unfortunately most people don't believe in good magic. They are less likely to believe in good luck spells than curses.
A TV show I was watching showed a reenactment of a true story. A man was dying because he was cursed. He believed he was cursed and that belief was killing him. To fix him the doctor pretended to use magic to break the curse. He was as convincing as the voodoo priest. Once the man thought the curse was broken he quickly recovered.