When I was around in middle school I did black magic but I never heard of the sin shirk or black magic when I heard the news I asked Allah for forgiveness and I sincerely promised not to do it again will I be held accountable and I took the shahada again
Assalam o Allaikum.
If you do not know that a certain act is prohibited in Islam then there is no sin upon you. In Islam, sin is based on the acts that we do willingly with knowledge of what religion says about it.
In Surah Al-An'am Ayat 54 Allah (subhanahu wa ta'ala) clears states:
When those come to thee who believe in Our signs, Say: "Peace be on you: Your Lord hath inscribed for Himself (the rule of) mercy: verily, if any of you did evil in ignorance, and thereafter repented, and amend (his conduct), lo! He is Oft-forgiving, Most Merciful.