I've had a quick look at the Wikipedia entry and my first thought was that it isn't the term "blood donor" itself which is the retronym, but the title of the episode, otherwise I can't see the point of mentioning it (since it's irrelevant to the issue) and especially the use of "though" after telling us the episode is one of the best-known** ie to me it reads as though the original episode title was simply "The Donor" or something of that kind, and they changed it retrospectively. However, if that's the case it's strange there's no reference to an original title in the piece, so I don't know.
It's possible "blood donor" as a term is a retronym if originally a person donating blood was simply a "donor" and the "blood" as a modifier was introduced after organ transplantation came into being -- ie organ donorship is very much later than blood donorship -- whereupon it was necessary to differentiate between blood donors and organ donors, but I've no idea if that's actually the case. We'd need someone with information about medical history and terminology to confirm the situation!
** Having said that, I'm not always convinced the people who make and edit the entries are actually au fait with correct English, so it might simply be an error