after killing that vampire bitch i found her black heart. so what
can i do with it ? is there a way to cleanse it or something ?

If you have Aerie in your party and she was killed in the conflict with Bodhi then you need the heart to heal her, take her body and the heart to the Temple Ruins - if you've already cleaned out this place then go inside and go the statue across the alphabet puzzle. Put both the heart and the body into the statue's arms and she'll be brought back to life.

I was stuck on this for a while!