Honestly, I could. It'd probably require deception, though. Once I found your address or even your name I can grab a list of your suspected relatives. After that, I can find their phone numbers, and call them, and tell them you're in the hospital and in need of a blood transfusion or you'll die.
They'll find me it very quickly.
I used intellius to conduct a background check on someone the other day and for a few bucks I got all their public records, prior addresses, emails they used when they were in high school (mine too, apparently AOL and Yahoo sell your data to companies like intellius). Also, a fairly complete list of relatives, and contact info for them as well, neighbors too.
While finding your blood type is definitely possible, I don't really have an interest in spying on you :)
That's cool. Part of the reason I asked is that I haven't even disclosed my email, let alone name, on HN.
re:family - my family is (unfortunately) well aware of aware that this would be a scam, having spent too much time with health services. Also, the fact that I wasn't raised in the US puts me at a distinct advantage here :)
But your point is well taken: some intellius&friends, some social engineering, and you get what you want.