I am blood group AB negative. In the old days I would have been one of the 'blue baby' ladies.
I have always known my mother is O positive, as she donated blood when she was younger and had a donor card.
For years I never knew what my father's blood group was, then just before he died, I saw this written on his hospital chart. I only glanced very briefly, and cannot remember quite what it was, but DO know it was not AB negative (I think it was O but I can't remember whether neg.