Assalamu Alaikum brothers in Islam,
I understand that black magic is considered as Major Kufr. However I don't understand why it is considered as shirk.
What aspect of black magic makes it Shirk exactly?
There are a number of reasons
Its because you rely and call on Satan and not Allah.
In black magic you preform certain rituals and acts for the worship of the jinn. Some magicians prostrate to jinns.
The jinn will tells you certain symbols and objects have certain meanings. Thus assigning meanings or knowledge to symbols and objects. These symbols and objects don't have these meanings/knowlege at all. But this lead to shirk and/kufr as you end up thinking that is how thing work (i.e. not through Allah). And end up as kufr, and/or worshipping jinn and/or worshipping these objects.