San Pedro is not an easy plant for people to send you black magic through. In my experience it is one of the few plants that doesnt have a dark side - it is more of a saintly/heavenly plant. It naturally protects you and its own spirit wont ever bring harm to others - it is a plant of healing and love.
However, it will show you when others have sent bad magic to you, and it will show you how to change that energy into something more harmonious. With San Pedro you dont ever fight black magic - you just take the energy someone sent to harm you and use that same exact energy to empower and uplift yourself instead. All energy can change forms - once someone sends you energy you can do whatever you want with that energy as long as you recognize it. In this way, if someone attacks you, you just thank them for all the momentum you can put into accomplishing beautiful things in your life!
San Pedro could also be pointing out a personal habit of always feeling attacked, or always seeing brujeria/witchcraft.... Most people dont consciously attack others with magic - though it can happen accidentally really easy. There is a trap you can get into though of always suspecting and looking for an attack - you can create the symptoms of an attack just like a self-fulfilling prophesy.
In my experience though, if you are really being attacked by sorcery of some kind, the best thing you can do is probably drink San Pedro. And if you are unsure about your seller, then find someone else. Plenty of shops on Amazon and ebay.
And whenever you are unsure about something in ceremony with San Pedro - ask the plant. It will guide you to the answers, or the lessons, or the right questions somehow if you keep asking it...