I mean listening to Hindu mantras with the intention that it will have some effect on someone or yourself. As far as I know black magic is a part of hindu religion, so I would like to know if listening to such thing with the intention that it might effect us in some way (positive or negative). If it is indeed Kufr or Shirk, then how does a person repent from it?
Recite At very First the kalma e tayyaba(1st kalma).
Then Ask Allah swt in Private and with all faith in him, "Oh lord Forgive me in the name of Holy Prophet Who is your Habib The one with guidance and Knowledge. I have commit the sin but I am feeling guilty and I will Never ever Perform it again Oh lord You Love your servant more than 70 times a mother does to her children . I seek refuge in You And there is No Gods To worship except You, Oh lord of Muhammad rasullahallah I seek Forgiveness".
Something like in this manner or other But never do it again Consider it a sin and a bad practice and seek his forgiveness because what Mine, Your and every creation's Lord says:-
"O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful. ... "O son of Adam, so long as you call upon Me, and ask of Me, I shall forgive you for what you have done, and I shall not mind."
If black magic is found in hindu mantras and you do not denounce it, you are committing sihr, and since sihr is kufr, you are also commiting kufr if you did not denounce it .
If you cannot denounce it by taking action, advise the people committing ths by talking, if you cannot then hate the sins the people are committing, and that is sufficient.
It is obligatory to denounce the evil by taking action, if you cannot then advise them by talking, if you cannot then hate the sin.