How do you get black magic power myself?

You cannot.

Magic is not real.

A Pagan Perspective:

You can not get "powers" you already have them.

Magic comes from within and is the manipulation of the energy that is around you to bring about change.

Also, magic is truly neither black nor white, it's neutral in an of itself, it's the intent of the caster that is used to determine whether magic is black (bad) or white (good).