It is not clear to me how the HUAC committee could "black list" anybody. As I recall it, those who were subpeoned to appear and did not cooperate with the committee were cited and some of them jailed. The most famous of them were the Hollywood 10 (I think it was 10). They were not "blacklisted" by the committee, though, they were "blacklisted" by the film industry.

So "who" blacklisted Pete Seeger?

I was in my early twenties during this time period, and kept up with events at the time, but haven't re-visited that time period for years, so my mind is a bit hazy on it.

DougR