It is not permissible to seek help of Jinns through black magic to harm other persons. Allah (SWT) says in Quran (And some men were seeking help from some Jinns but the Jinns added on their difficulties)Sura Al-Jinn, Verse 6. This means that seeking help of Jinns is bad and harmful.
Harming others through any way is also a sinful act. So, the aim of harming others is out of Muslim morals and the mean of Jinns is also bad.
'Wassalam.