Neither of us were there. I don't know where you grew up but people in their late teens/early 20s did a lot of weird stuff in Los Angeles where I grew up and it does not, looking back, serve as much of a predictor of future thoughts, feelings or ideals. (Taking drugs or engaging in promiscuous sex is different and this is not indicated here.)
Since I don't believe that witchcraft works I don't see any lasting damage from having sat upon a stone with some blood on it.
I think an argument could be made to the effectiveness of ritual. I think it has been made, actually.