OP I was googling my own birthmark a few years ago and I recall coming up with a couple of hits on this, it was supposed to be a red birthmark in the shape of a cross or heart depending on which site you read and located on the shoulder blade back or heart again depends on the site.

Thing is if it was a red birthmark, like mine, also known as a strawberry or Hemangioma birth mark, these fade in pretty much every case before you hit your teens and are not past on genetically. Hope this helps.

Sorry OP I meant to add this originally, it came from Deuteronomy 33:12 "and he shall cover him all day long and dwell between his shoulders" Apparently this is how Benjamin was marked and some researches interpreted this to be the birthmark.