My theory is that Black Jack travelled to the future :) Claire keeps talking about Inverness - Black Jack might just follow her there, and to Craigh na Dun, and he might touch the stones if he sees her doing that. He could be sent to another point in time - before Claire and Frank met. Then, he just looks up Claire Beauchamp (smart, Claire, telling him your maiden name), seduce her (of course he would totally be capable of that), marry her and regale her with tales of historic events (which he actually witnessed firsthand).
