Assalamu'Alaykum
Allahu'A3lam......
yes Allah is the most forgiving... there is a difference with black magic, because u are harming a person, and when u harm someone Allah will not forgive u till that person forgives u, this is called Huqqel Ebad.
The rights of the servent. Same as backbitting and opression...these are between u and the servent of Allah. Things that have to do with u and Allah... ALlah forgives...such as shirk...but Blackmagic this is harming another creation of Allah...but it is said that if and untill the servent that u harmed forgives u then Allah will forgive.